What do I do about my fianc�?

Ok so I'm having issues with my fianc�, we live with her parens, and for the last month now we get up, and she heads right upstairs to her moms room, and stays up there until midnight if not later. We usednto go out and do things but now I'm lucky if I get to see her even twice during the day. I've talked to her about it and she gets really defensive and she says I'm being clingy and smothering but how can I be when I don't see her for 16 hours out of the day! It's also talking a toll on me in the bedroom, I get so mad at her for ditching me and basically ignoring me that when it's time for us to have sex (which is like once every two weeks) I can't even get it up. She said she misses me and thinks about me all day while she's up stairs and that spending all this timewith her mom is 'what she needs right now'. I don't know what to do and it's starting to really affect every part of our relationship. The other part is her friend lindzi, when this chick is around it's nothing but trouble, I become the third wheel, I'm ignored, and my fianc� turns into somethingf completely different and holds her hand instead of mine, walks with her instead of me and spends WAYYY more time with her then she's does with me or even with her own family. Again when we talk about it I'm being clingy and she get defensive saying that 'they're close as sisters' and that she is her other half but 'in a different way', she also involves her in EVERY aspect in our relationship and even want her to live with us, this girl is a leach and hosts on one person after another and my fianc� doesn't seem to see it, and if she does is in denial about it. I'm at a loss here and have no idea what the he'll to do. I am not going to leave her and love her more than anything, I've given everything for this girl and know I want to spend my life with her, but I want our relationship back