My dad grew up in a foster home and didn't meet his older brother until he was an adult. The last time I saw them together, it struck me how similarly they acted. Yes, they did grow up in the same city (and in fact only a couple of miles apart, just enough that they went to different high schools), but it's more than a common regional accent and culture; they were similar in a way that Dad's other brother (who was in the same foster home as Dad) wasn't.

When my ex and I decided that our kid would move with ex back to Belgium (better social services for kid, whose autism is strong enough that they can't live independently), I had a brief thought that I was taking after my paternal grandmother, who left her first three kids with her parents and her last two in an orphanage; on the other hand, my marriage history is a lot tamer than hers. A year ago I met my second cousin, descendant of my grandmother's sister; the sister had also left her children to be raised by other relatives, and had married and divorced several times. Definitely makes me wonder how much of that was culture/home environment and how much was inborn temperament.