If your sibling was obliviously in love with another sibling, would you tell her

Sorry, I should have been more specific. I meant to address this caveat—

Why was this necessary? There’s no real problem with first cousins having children.

Because people have hysterectomies for lots of reasons. Like cancer.

And some people aren’t walking founts of knowledge like Dopers are. Maybe she is lying about having had an hysterectomy just so that people won’t bother warning them of the reproductive “dangers” in their future.

But we are actually talking about siblings, just in case you missed this critical piece of information.

I would tell her. In a general sense, I’m just a fan of information being shared. In fact, it’s unlikely I’d be holding onto this family secret to begin with, because I feel like some of the people affected should know the truth at some point in their lives when they’re old enough to handle it. The secrets I keep from a ten-year-old are not things I’d keep from a twenty-year-old.

Also, I don’t want her to discover it later. I have no problem with siblings marrying (especially with the reproduction issue out of the way), but maybe one of them would have a problem. Better to know about that problem now before it gets any more complicated to unwind the relationship. I especially don’t want her to discover it later in a way that makes it clear I knew all along.

I would tell her the truth. Not to fight the hypothetical, but I would have told my sister about our brother as soon as I found out about him, because she would have the right to know.

I think you’re right. Similarly, you can’t marry your step-sibling if he/she is legally adopted by your parents, and is, legally, your sibling. Ditto an adopted sibling.

As for the OP… I’d say let it go. Why ruin someone’s happiness for no real reason?

I would tell, not because it’s any of my business, given the info in the hypothetical, but because if Auntie told me, there’s a good chance she’s going to shoot her mouth off to someone else down the road. Heck, maybe she already has blabbed about it and that whole side of the family already knows, or at any rate, “knows”, or thinks they know. Sister could be heading for a giant, public, minefield and she deserves a heads up.

I probably wouldn’t talk about my own snooping. I’d keep it vague and then the ball’s in her court for how she wants to handle it. I’d say something like, “Look, before you make an announcement, you should know that Auntie Looselips once told me that Daddy was a bit of a tomcat and that Cousin was really our brother. I don’t know, she’s kind of a flake, but - that’s what she told me and I don’t know if she’s told anyone else yet or not.” Then I’d let her take it from there.

Why would he have been adopted by the aunt? She’s his biological mother.

Well, “your” aunt/the guy’s mother obviously knows, and what’s more she knows that you know since she’s the one who told you. So it’s entirely possible that the happy couple will eventually find out from auntie that you knew the truth all along even if you keep quiet about it.

My initial reaction to this part is “fuck them and what they think” (and note that this is almost exactly my position in my family: I don’t feel that it’s a bit of their business - if I were to date someone seriously, yes, they would meet them, but until that point, I don’t feel my love life is something to share with them. I do know that parts of the family speculate about my sexuality. Whatever. I’m not, however, dating a first cousin of any sort, much less one who might or might not be a sibling).
As to your actual scenario: I don’t know. I think I would want to know, but I don’t feel like I’d want to be the one to tell - because I don’t want people to end up hurt because of information I shared with them. But if I felt like I was the best person to do it, I would, though I’d be very careful in how I told her.

Either way, I still think they have a right to know something that big.

Assuming arguendo that you’re right about its being illegal, how the hell will anyone in a position of authority know it’s illegal, unless some loudmouth challenges the legitimacy of the marriage?

True, they have a right to know. And they have just as much of a right to not know. But once they know, you can’t give that latter right back to them.

IMHO, their right to know is their right to expect the truth from people if they inquire about it. It doesn’t create an obligation for others to tell them if they aren’t seeking knowledge on this subject.

I had a question about that, is it that:

  1. The father had an affair with his sister-in-law, resulting in a baby

or

  1. The father had an affair with some random woman, resulting a baby that she dropped off on his doorstep in the middle of the night, and the father thought “hmmm, maybe my sister and her husband, who desperately want a baby but cannot conceive, would like to adopt THIS baby and raise him as their own.” Win-win!

In the first case, there’s more of a blood relationship, as the couple would be related on both the mother’s side and the father’s side.

Anyway, neither scenario would matter to me, I would keep my mouth shut and let them live their lives. My general rule of thumb for situations like this is that unless I have seen the events unfold with my own eyes to treat it like it might possibly be a rumor, because really, who wants to be in the middle of that mess?

That would be right firmly in the “mind my own business” camp.

I mean, honestly, who knows if it’s even true? I mean, yes, auntie screwed my dad, but surely she was also having sex with the legal father, since he accepted the kid as his own? Or does everyone in the family know expect sis and cousin? If so, I really don’t see why I should be the one to tell them. I don’t think anyone should.

I think I’d want to know, but I don’t think I would tell them. I would want to know because that’s disgusting and horrifying to me and I’d want to stop the madness. BUT, I would never get with a cousin either, so obviously they are not like me. Really, between first cousin and biological half-sibling who you weren’t raised as siblings and don’t even know you’re siblings AND inbreeding isn’t an issue…well, it’s really not that much worse.

The OP says that the aunt is the guy’s mother. If this were supposed to be some more complicated scenario where the aunt adopted a boy who just happened to be fathered by her own brother or brother-in-law then I assume the OP would have said so.

The aunt is not necessarily a blood relative – she could be the wife of “your” father’s brother. But I think it’s the half-sibling thing that concerns people; whether or not the guy is also a cousin by blood seems like a pretty minor point compared to that.

Other people already know, so it could quite possibly come up.

I realized that I didn’t actually answer the question being asked here. Since there are at least two (me and Auntie) and probably three or more (Dad, maybe other relatives and friends) people who know that Sister and “Cousin” are half-siblings, it seems likely that the truth is going to eventually come out one way or another. Better that Sister and Cousin find out before the wedding than afterwards.

However, while I do think they should be told the truth I don’t think I’d be the best person to tell them. It would be better for Dad and/or Auntie to fess up. I’d probably go to Auntie first (since she’s the one who blabbed to me about this in the first place) and explain about the pending elopement. If that failed I’d go to Dad and tell him that I knew about his fling with Auntie and that Sister and Cousin are about to elope.

Yeah, but that’s still a long way from tracking down where they eloped to, and presenting sufficient evidence to the courts in that jurisdiction for them to open a proceeding to potentially invalidate the marriage license. I expect that the person who took that initiative would need, at a minimum, a notarized affidavit from Dad or Auntie detailing this bit of family history.

The likelihood of this is probably next to zero, given that while Auntie may have told you for some reason, she hasn’t told enough people that this has become general family knowledge over the intervening years, and neither has Dad. It doesn’t seem like either or them would be a party to sharing this knowledge with the court system, unless you already know one of them isn’t exactly happy with their imminent marriage.

monstro: a key bit of this hypothetical scenario that’s not in your OP would be, how did Dad and Auntie react to the news? Are they even still around? Dad’s the one who’s got two of his children about to get hitched, and Auntie’s the mother of the groom. If they’re good with it (or deceased), it’s not for you to spill.

OTOH, if they’re clearly uncomfortable with this turn of events, then they’re the ones you want to talk to. If the news should come to the happy couple from anyone, it should come from Dad and/or Auntie in a private conversation with your sister and cousin/half-bro.

I’m also curious as to whether Auntie was the bio-mom or adoptive mother of your cousin/half-brother in this hypothetical. In the former situation, ISTM that the only way you could confirm this would be confronting your dad, not through any sort of snooping. If the latter, snooping might turn up records of where your dad signed the papers terminating his parental rights to the child.

Let’s say the dad’s not around anymore. The aunt is. And the aunt is the bio-mom.

I agree that it wouldn’t be wise to assume you are the only person who knows this information. If someone else knows (and we already know the aunt does), then that means there’s a good chance the truth will come out. But I don’t think it should be you who necessarily does the blabbing. If there’s any emotional backsplash from the revelation, it should end up on your aunt, not on you.

But let’s say the sister has sworn you to secrecy about her relationship until they elope. That means you can’t go to the aunt and tell her exactly why she needs to come clean. That would be a difficult spot to be in. So I’d probably go to the sister and the cousin/brother and insist that before they do anything, they need to reveal their relationship to Auntie. If they balk at this, then I dunno what I would do. I guess I would tell them what “I’ve heard” and let them go about the trouble of confirming it. But I’m not sure I’d want to be in that position.

OK, that helps set the stage.

That’s a new spin. Admittedly this doesn’t directly contradict anything in the OP, but people who are like this

are generally telling the whole world about their new romance and how happy they are. So I was assuming the whole family knew about this cousin, cousine romance.

So reviewing the bidding:

Dad’s dead, Auntie is alive. Cousin/half-bro is the offspring of Dad’s fling with Auntie, his wife’s sister.

There are only two possibilities:

  1. The time Auntie mentioned this to you is the only time she, or your dad, ever blabbed to anyone about it.

  2. It’s not the only time.

Maybe there’s a sub-scenario of (2) where just one other person besides you and Auntie knows, and they’ve kept their mouth shut all this time. But beyond that, Dad’s and Auntie’s fling has to be the scandal of the family that everyone knows about, and the possibility that Cousin is really Dad’s son is maybe not nailed down, but everyone knows it’s a possibility. And Mom ditched Dad years ago on account of the affair, hasn’t spoken with her sister in eons, etc.

If Dad’s and Auntie’s fling isn’t already well known, then there’s really no reason to believe it will become so at this point, and there’s no reason to spill the beans.