I agree with what Lezlers said. She may be an adult in years, but not in actions. Lupine73, your girl has made it clear what her priorities are, and at best your in third place and a video game is way ahead of you. It sounds like your living arrangements are tolerable for now, but as soon as you can manage a place of your own, I’d move out. She’s not going to change; she’s got no reason to. It sounds like you, on the other hand, have a rather nice, full life. Leave her, move on, and find better. You deserve it.
I think that’s part of the problem right there. It sounds as if, to her, you’re a roommate who she also happens to date. You’re looking at it as a committed live-in situation, which I gotta tell you, doesn’t happen that often in a house with mother and sister. Of course, if you don’t have your own room, and only sleep in her bed, my suggested viewpoint for her is a little harder to support, but we’ve already established that this girl doesn’t have the clearest grasp of reality in the world.
Yeah, get out as fast as you can. Screw inconvenience, screw expense. Just get out. This is NOT a good situation. Well, maybe if you offed the girlfriend. You seem to have a wonderful relationship with the mom and sister. 
Yeah, I’ve made that decision… I’m just saving up as much as I need to, getting an apt. and getting out as soon as I can. I’ve got all the essentials (bed, couple dressers, computer desk, etc)… I’ll furnish the rest of it as I go. Need to figure out about how much I’d need to save up to take care of the essentials (deposits, moving truck, phone, electric, internet… etc)…
In the name of all that’s naive or, at least, absurdly optimistic… maybe if she shows some kind of change once I’m out of there… maybe things could turn around. She made a comment, apparently, to her sister - something about “maybe it’s that I don’t have a chance to miss him”. Not an unusual or unreasonable sentiment… except that she was all for me moving in in the first place.
Nonetheless… moving out will at least put some distance between us, give me some more space to be myself and breathe a bit more. It’ll also eliminate that element from why she might be acting how she is. If nothing changes after that… say within a couple weeks… then I’m just ending it. For the record, I’m giving it the benefit of the doubt and will give it that one more shot… but I don’t expect it’s going to be any different. Could it happen? Sure. Do I expect it will? Signs say “No”.