
Maybe they worry that if you just hang out with your sibling/cousin all the time, your horizons won't expand. Meeting different people with different view points is important for mental growth.
Or... There was an incident much like what's going to happen here that caused a scandal and new rules were put in place.

You all thinking about sexual/social problems but its probably because blood relatives fight for real all the time, physically. You dont do that to strangers, but holy shit me and my brother, he is 2 years older than me, it was pretty much UFC 24/7. We love each other btw, we would made up 10 mins later still, its problematic for the school.
Totally unrelated to everyone's concern but why would blood relatives be forbiden From sharing a room???