True, but usually they are not treated like that. If, IDK, a father and a son have a kid at the same time (with different women, you pervs) usually they are raised more like siblings. Yeah, they are uncle and nephew, but having the same age, grow up together and being from the same generation make that.
Damn, my brother has nephews older than him, is not so strange today.
In this situation, being all the kids from the same age more of less is just like having a lot of moms and a dad, even if they are cousins, brothers, sisters, uncles, etc.
Strange? Fuck yes. For the kids? Not so much, not until they grown up, and after years with that situation is just what it is, dad is a perv and the moms are weird, especially mama Nova, but it's okay, she plays videogames with us, its a cool mama.