I would say never
There is no law about that.
There is no law that says a brother of a certain age can not share a room with his sister.
It's legal if they're brother and sister, maybe.
it is not illegal for a brother and sister who are aged 5 and 2 to have to share a room in New Hampshire. The parents can however have them in different rooms if the space permits.
Latios and Latias is brother and sister Latias is the sister Latios is the brother if u seen the movie they share the same father
brother and sister
Well there are some laws that say a brother and sister can't share a bedroom, but I'm pretty sure a boy as young as 2 can. When he gets older you'll have to check the law where you live.
Of course it is legal for siblings of the same sex to share a bed! Not everything in life is legal-illegal--- thank goodness! Plenty of siblings shared a bed growing up and are none the worse for it.
Your brother's wife's brother is your brother's brother-in-law and your sister-in-law's brother. But he is not considered to be related to you. Usually you could just refer to him by his name.
It is not illegal for siblings to share a room. Age has nothing to do with it.
... will share a birthdate.
I imagine you are talking about your spouse's sibling's spouse (your wife's sister's husband). No.