I don't think my boyfriends sister likes me, what do you think?

Anonymous
Sorry for the long story. I just don't know what to do.

My boyfriend and I have been together for almost 2 years and are getting really serious. I am a big time family girl. I think that it is important that whomever I marry that their family will like me as well. My boyfriend agrees. Since day one he has wanted me and his sister to become friends. However, I have been feeling mixed vibes from my boyfriends sister. We live hours away but we still communicate over the internet or phone. She visited us a month ago and I felt like we had bonding time. We actually had time to get to know each other. However, off and on for no apparent reason she acts irritated with me like she hates me. She is always hot and cold with me. She has been the same way with my boyfriend. He called her today and she was very snappy with him too. We are both very nice to her. I don't know if it is her personality or if she really just doesn't like me and him being together. She is always hanging out with his ex of 5 years. The one that wanted him back and probably still does. It bothers me that every time I feel like we are becoming friends she does a 180 for no reason and is very moody or ignores me. This girl is a grown woman in her early 20s but sometimes she throws fits when she doesn't get her way. How do you become friends with someone who acts like they like you one minute and hates you the next for no reason. She is the same with his ex. She tells me that she can't stand his ex but the next minute we hear that they are hanging out a lot. I just listen and never respond to her complaints about his ex because I don't believe in trash talk behind someones back. I don't know what to do because she is my boyfriends sister and most likely future sister inlaw. HELP
I don't think my boyfriends sister likes me, what do you think?
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