11 year old moving in with us and our infant?

We have a 9 month old baby, live in a two bedroom apartment, and due to family issues an eleven year old girl needs to move in with us for a while. I'm happy to help and I think it would be good for her because she comes from a difficult situation and we have a very structured household, but I'm apprehensive because I worry about keeping my LO safe and healthy and also I'm just not very experienced with her age. Any advice or thoughts? Posting anonymously in case any family is on here :)