Can my boyfriends 6 year son sleep in the same room as my 11 year daughter They will be in bunkbeds She has told is its against the law. We are in California they are in Arkansas

Who is 'she?' Your 11 year old? The mother of the 6 year old?
No, it isn't against the law except in some public housing, where rules are set locally, and often do require separate bedrooms for each gender, as well as have limits on how many can share a bedroom or apartment regardless of gender.
I see no problem with it, but your daughter may be at the 'yucky' stage about this sort of thing.

If your daughter (or anyone for that matter) says something is against the law, then ask them to cite the statute or rule that makes it illegal. Anyone can say its illegal.

If this is your daughter that says so, its probably because she is at the stage where her body is developing and her privacy is important to her. And you need to be sensitive to that. But if there is no other alternative, she needs to understand that as well.

If it's the boy's mother ask her why it concerns her.

By the way if this is your boyfriend, then the child is not your stepson. If and when you marry your boyfriend, THEN he will be your stepson.