I guess I think extended family ends about at the point where I would consider dating someone if they were suitable.

So, probably around 2nd/3rd cousins. I wouldn't freak out (much) if I found out I was sleeping with someone I shared a great grandparent with. Or a great great grandparent. That's a distant enough relationship for it not to be weird if it blows up-- the old people are the only ones who'd care even slightly-- plus doesn't pose any serious genetic issues if a kid resulted.

My in laws have this enormous annual family reunion, originally driven by a large group of siblings and their descendants.

At this point the original sibling group is getting old and tired-- the elsest are well into their 80s-- so a few of the second gen (50somethings) are trying to step up. But really, it feels forced, like 150 randomly selected people are having to spend a weekend with each other to please grandma/grandpa.

Especially once you start getting into the third and fourth generations. Those people have hardly anything in common.