Why is my girlfriend projecting her sisters failed marriage on our relationship?

I have been going out with my girlfriend for a year now, we spend most of our time together sleeping at each others place, in the past week she had been pushing me away, saying she just wants to go home and that she doesn't want to spend the night witch we did all the time spending a lot of time together has become the "norm" and its something she wanted to, but now that her sister who she looks up to and wants to follow in her footsteps decided to end her marriage to her husband and is saying all this stuff to my girlfriend (her sister) about how he is cheap, annoying and didn't treat her right. we talked and she told me how she is kinda projecting that relationship on ours witch has nothing to do with ours. she is getting really confused and she dose not totally understand why she is pushing me away. she says i am a great boyfriend and she dose not want to lose me but i don't think it is fair that she is projecting her sisters failed marriage and relationship on ours. what should i do?