My sister has a 3 year old son and 1 year old daughter and a baby girl on the way. She has a 2 bedroom low income apartment
There is no determined legal age. It is a matter of Judgment. A three year old and 1 year old are still young enough that there should not be a problem if they share a bedroom. However, as they become older and closer to puberty, there will be a problem. i it sounds as though the home may become crowded. If it becomes too crowded, she may need to rearrange the sleeping arrangements or seek assistance in finding a larger apartment.