Is it legal for boy 11 & girl 3 (not siblings) to share a room?
The legality of it really depends on where you live. Different states have different laws concerning that. Honestly though, they really need separate rooms. The 3 year old is at an age where their curiosity about their own body and the differences between the sexes is getting ready to peak. The boy is about to start exploring his own sexuality in much the same way. All that aside, the boy is going to constantly get upset because his sister (step-sister and sister are the same thing… try to make a whole family here) is going to be getting into all of his stuff. For your own headaches, give them separate rooms.
