No, it’s not.
I don’t think I’d be inclined to approach it any differently than any stranger wanting to get to know me, aside from perhaps letting my mom know this person showed up, in case there are any legal issues that would need to be sorted out. (Well, and, if they claimed to be my mother’s child, I’d laugh and give them the boot, because I know my mom well enough to know that that had no chance to happen. And I don’t make friends with dishonest people.)
Whether or not we’d become close would depend entirely on whether we have anything in common. I have not a damn thing in common with the sibs I do have, so we’re not friends or anything and don’t go out of our way to talk or hang out aside from winter holidays, where we do family stuff mostly for the sake of my mom. I don’t dislike my sister, and do honestly enjoy our once-a-year snowmobile outing, but we’re really not involved in each others’ lives because I really don’t grok what she’s into and she doesn’t grok what I’m into. We have basically nothing to talk about, so a once-a-year conversation to catch up on each others’ news that we don’t grok pretty much covers it. We’re not friends by any means, and if we hadn’t grown up together I doubt we’d be in touch at all. I don’t even think she realizes I’m pagan, despite the fact that it’s never been a secret, ever. Funny story: in high school (ehr, ME in high school, she’s older), she asked me to be my nephew’s godmother; given that I’d been begging Mom for years to let me stay home from church on Sundays, to no avail, I figured that everyone knew I wasn’t interested in Catholicism, and that this was a token honor. It wasn’t til several years later that Mom and my sis mentioned in passing that this nephew’s godfather was not religious, but it was okay because you really only need one of the godparents to be Catholic. I was speechless. :eek:
So if this person shows up all wanting to be best buds evar, I’d just tell them to slow down and see if we have any cause for being best buds, first. I’d certainly talk to them, but wouldn’t commit myself to anything beyond that, unless and until it was evident that we get on like gangbusters. If they fit within my circle of friends/chosen family, of course they’re welcome, as anyone would be. If not, I’d certainly wish them well, but wouldn’t go out of my way to keep up contact or anything. I don’t see the point in chasing after awkward conversations. My chosen family has nothing to do with genetics.
This is actually pretty similar to a cousin I met in adulthood. We had dinner with my aunt and uncle, and this cousin’s parents who are (I believe) my uncle’s cousin and his wife. It was pleasant enough; they were in town on vacation and had come to see a show I was in. My cousin lives only a couple miles away from me, and we’re Facebook friends, but we’ve had very few conversations since then. I don’t believe we’ve seen each other in person since then, either; I invited him to a few things where he said he’d maybe show up and didn’t (pet peeve of mine); so eventually I just didn’t bother anymore. We don’t dislike each other or anything, but he’s not really on my radar, and I doubt if I’m really on his.