Opposite gender siblings in same room, illegal?

We live in a 3 bedroom home and don't want to go any larger. I have a daughter who is 6 almost 7 and a younger daughter who is 16 months, my husband and I are ready to have our 3rd child and are currently trying to conceive. I mentioned to a friend of mine that I will put my youngest daughter and the baby together in a room and give my older daughter her own room because of the age difference. My friend said, "unless it's a boy, then you will have to put the girls together and give the boy his own room, it's illegal to put a boy and girl in the same room" This seems completely ludicrous to me. I know if you take in foster kids you have to have a separate room for boys and girls and I can understand that. But these are all my own flesh and blood children who are full siblings. Is she right and it is illegal or is she completely nuts?