Move out or Stay home

My boyfriends grandmother is giving him her house. She has a boyfriend and they have been dating for a while. Her boyfriend is buying a new place and asked the grandmother to move in with him. She has her own house but won't sell it because if things don't work out with the boyfriend she wants her house back. My boyfriend asked me to move in with him when this happens. I can't go back to my parents house if I do move out but if the grandma does decide to come back I can't stay there anymore what would you do?