That's a hard one with no real answers. When you're at home, you're just roommates doing things that roommates do, and I think that is the right way to be though I get how awkward it could feel. She doesn't have a claim on him, I can tell you that.

You need to plan time away with him....go on a date as you would with anyone else you liked. Maybe you two could discreetly text each other while at home things you'd like to tell one another and keep from the other girl but in a way she doesn't get you're texting one another.