Hi everyone, my name is Nikki. I'm 22 and I've been with my fiancé for 5 years. We recently became convinced that his sister has BPD. She had always done things that disturbed me in their level of irresponsibility and selfishness, but my fiancé always just told me she was very spoiled and manipulative.
But about a month ago I became the target of her outlandish rage. It started on Facebook. Apparently there was a post on my fiancé's page written by his friend stating that a particular political point of view was "cowardly". His sister just so happened to believe the so-called "cowardly" point of view. Now I occasionally make political posts on my Facebook and clearly she'd seen the ones where I expressed a different point of view from hers on the subject, but I had nothing to do with my fiancé's friend's rant. All I knew is that one day she posted 4 pictures on my wall from her political pages expressing her views. At first I was really baffled (one of them said "Hitler get 6 million people to follow him blindly? Then I realized I'm in America." What does that even mean?), but I figured she must be trying to get on my nerves for some reason. So I decided to "kill her with kindness" and respond to each one in the nicest way possible while explaining why I disagree. I told her I enjoyed discussing politics and that I hoped we could learn from each other. She responded with different variations of "it's cute how naive you are

" and told me I was a "sheep who believes whatever the media tells me" and etc. I responded by telling her that I was interested in hearing her opinions but I wasn't going to talk to her if she just wanted to be angry and call me names.
This is when the s hit the fan. She said she never called me names and that I was the one who called her a coward. I had NO IDEA what she was talking about. I was worried she was on drugs so I texted her mom (with whom she lives) to ask if she was ok because she was posting weird things on my Facebook that didn't make sense. She said she was fine but under a lot of stress. My future sister in law then sent me a private message saying all kinds of things about how I think I'm so much better than her and I have never cared about her and called me names like "know it all btch" and so forth. So I took screen shots to show my fiancé and blocked her. After I showed him he logged on to his Facebook and discovered a comment she left before she started things with me on that (month old) post saying things about how vegetarians don't know anything about the "real" world, when eating meat had nothing to do with anything. I am the only vegetarian around, so she clearly somehow thought she was talking to me.
He then saw her status saying, "A certain someone called me a coward for believing x and told me I was ignorant!" Her mother in law asked her what happened and she replied "I'll message you... . " So my fiancé posted telling her that she needed to pay attention to what she was doing because what she read was on HIS Facebook. So she did the only rational thing and blocked him and started to text us both constantly for over an hour. The texts she sent me were just a series of insults, and ironically demanding that I leave her alone despite the fact that I did not respond to a single one. The texts she sent him were mainly about how "blood is supposed to be thicker than water" and he loves me so much more than her and it's not fair because her life has been *so hard* while I have never had a real problem in my life.
After the smoke cleared he did some Internet research and discovered BPD. It explains everything, including behavior she displayed in childhood. My problem right now is how to move forward. I'm still hurt and angry because I feel like I can't go to family functions anymore, particularly because her in laws are at every one, and who knows what lies she told them about me (she went to stay with them for a few days while she was still upset because her mom wouldn't take her side 100%). I really want to be a part of my fiancé's family, but I don't know how to deal. Help?