First, she should & need to respect that it's your sister's house and not her own child's house. Does your sister know that she'll be sleeping over? Must be an awkward moment since both of them has no relationship & nothing to do with each other and will keep bumping with each other everyday. Secondly, where will she be sleeping in the room? On the floor? Just to take care of you and baby? Best to talk to your husband and tell him how you feel. Of course you need your own privacy too. Will he like it if your mum invade his privacy and sleep together in the same room?
sorry to say this sound so wrong to me even her intention is to help. but I think privacy is very important.