Argh! Look at the date!
The OP is still around, so maybe we’ll get an update. It’s an interesting situation.
For what it’s worth, my take on it was that purple haze didn’t like his brother’s girlfriend for a variety of reasons, not necessarily unfounded, and decided to focus on the second-cousin aspect as an easy target of his disapproval.
We can never know for sure, but I’d wager that if she were a good person and he liked her, he’d have been less grossed out by the fact that they were semi-distantly related.
Now I’m curious to know how it’s all shaken out.
Americans somehow have this awfully weird taboo against cousin marriage. But I really think it’s perfectly fine, even for first cousins.
The problem comes when the pair breaks up, because they’ll still potentially have to see each other at family events for the rest of their lives.
Same is true for anybody who has kids. You deal.
–Cliffy
Question: I have a first cousin. Are my kids her second cousins or her first cousins once removed?
Your first cousin and your kids are first cousins once removed. Your kids and her kids are second cousins.
I don’t think I’ve ever met any of my second cousins, nor do I know if I have any. I guess they would be my parents’ cousin’s kids? I have no idea if they exist. If so, (and were I not so happily married) I’d have no qualms about dating them.
Rhiannon answered, but I wanted to add that “removed” refers to differences in generations. Once removed means one generation apart, ie: your grandparents are their great grandparents or vice versa.
Something very similar happened to me with my best friend from childhood. I still consider him to be one of closest friends, even though we haven’t met in person for 25 years (immigration) He got married 15 years ago, and I just found out his wife is his cousin. But he is in Pakistan, and in some communities there, this is quite common. In their particular sect/ethnic group there are some more complicated rules that forbid some kind of marriages in the next generation after a first cousin marriage. I think his kids cannot marry some of their cousins.
I was squicked out a bit when I found out. But then I remembered that I have been out of Pakistan too long. Many things that were perfectly normal there, seem quite weird and scary to me now.
Right, but your kids are/would be second cousins to his kids.
I find it easiest to think of it this way: second cousins are the children of first cousins. My parent’s first cousin is my first cousin once removed, but my parent’s first cousin’s kid is my second cousin.
The great-grandfather thing I find confusing.
Doesn’t really bother me; in fact I sort of did it myself. I don’t really know what sort of ‘cousin’ this would be considered but here is the breakdown:
My first cousins on my dad’s side have a mother who had a sister. This sister had a daughter and this daughter and I dated for a while. We actually didn’t even know about our distant familial connection until after we had started seeing each other. It was a bit jarring to find out at first but mainly because of my own hang-ups; nothing innate to the relationship itself. We basically had no blood relation.
Unless I’m misreading your breakdown, that’s not any sort of ‘cousin’ at all.
Well I didn’t know what to call it. But it is some familial connection. And I felt weird about it. For a minute. But you’re right, I’m sure. What would the proper name be? Nothing at all? 
Your cousin’s cousin is not your cousin.
My first husband’s cousin’s mother’s uncle married my father’s aunt’s mother-in-law’s sister. What did that make us? Absolutely bloody nothing. But it means two of his cousins and some of my dad’s cousins share a mutual set of cousins. It also reminds me of that one scene in Spaceballs.
An update:
Well, the relationship ended abruptly two years ago. He came home one day and she was gone. He waited all that night and part of the next day, worried, before the neighbor told him that she had moved out. She left nearly all of her possessions behind and took off without a word. It hurt him very badly and he still hasn’t recovered from that entirely. He hasn’t dated since then.
From what my brother has heard since then, she had been seeing somebody else and as soon as she had another place to go she bolted. She did have emotional issues and really messed with his head.