Thank you so much for your replies!
Rose Tiger: That article and the comment were so helpful! That was my feeling that telling her wouldn't really help because she is constantly saying how her life and her anxiety and all her problems are so bad because of her husband because he is not perfect. So her thinking is that everyone else is the problem and not her.
justnothing: I did not think to have the therapist tell her though and I think that is the best option. The problem is getting her to see one.
To explain a little more: from the time she and my BIL started dating, they were constantly together after the first date. Rarely did they spend time around us or the rest of the family but when they did she was very clingy and overly affectionate to the point that it made us all uncomfortable. She was always kind and very sweet to us but would not leave his side. Once they got married, she refused to come over to my inlaws or any family function for several months. Since then it has got a little better only in that my BIL put his foot down and said they will come for the first sunday of the month dinner. But unless there is another family function they don't come over. She will not let my BIL text or call his family once he is home from work, he is not allowed to go do anything with the guys (his brothers), or his family except for the first sunday dinner or family things planned in advance. My BIL grew up hunting and every fall his brothers and dad go hunting, she refused to let him go the first year, the second year she was pregnant but let him go and then told me that she had to keep texting and calling him to "make him feel guilty for leaving me". This past fall she let him go but her mom stayed with her because she is terrified to be alone. After about a year we learned she has a severe fear of germs so if anyone has a cough, sniffle, runny nose, or upset stomach, she refuses to come around (that was okay, we understood that).
She has always idolized my BIL, always saying how good and perfect he was (not a bad thing by anymeans

) but recently in talking to her she told me about some major marital problems they are having. To help explain, we are LDS and have specific views about pornography and avoid viewing it. After she had the baby she has been more anxious and for the first few weeks had severe post partum but then just snapped out of it (that's according to her; I think she still has something lingering, having suffered with PP, depression, anxiety myself for years). During that time she refused to let anyone come over and if someone did she refused to come out of her room and see them. Now she is the classical overbearing mom (that too though is not the worst thing in my opinion, how many first time parents don't hover?). So I guess I'm trying to say that in the time since she had the baby their marriage has been more difficult and there had been more fights. She admits to having a bad temper but has said she thinks huge fights in a marriage are completely normal and your marriage is not normal if you don't have huge fights with yelling, screaming, threatening, name calling. The problem is that she gets over it fast but it has left lasting scars and hurts for my BIL.
About 5 months ago she caught my BIL viewing pornography and lost it, she smashed the lap top, trashed their house, sold nearly all their furniture, including the table set my inlaws bought them as a wedding gift. When she was talking to me all she could say was how awful of a person he was, how he was so terrible to her, did it to hurt her, that she wished he was an alcoholic, that she wished he had had an affair instead, that he never puts her first (which is not true, in my opinion he is the most attentive, selfless husband i've ever seen. Once he is home he takes complete care of the baby, gets up with the baby EVERY night and morning, has nearly cut off all contact with his family for her,etc.). Of course I understand that there is going to be hurt and anger from her discovery but she seems to have taken it very hard. To make a long story short, she constantly threatens to leave then the next minute is saying how much she loves him and being very intimate and affectionate in front of us. She bounces back and forth so fast. My brother in law has since said he feels he has no freedom and has no self worth left. She treats him like a child, not letting him watch any tv or use a computer, he can't go anywhere with out her or be left alone. She also has some very major insecurity and self esteem issues and I think they have gotten worse since the baby. In my opinion I believe that to her, finding out he was viewing pornography was a personal blow to her, saying she was not pretty enough, did not have a good enough body, that he has always hated her and wanted to hurt her, that he has always lied to her. I think that it just validated all her fears and insecurities (in her mind) and she can't get past that. Since that time she has blown up in front of my inlaws, accusing them of making my BIL so awful, telling them to get rid of their tvs, computers, etc. That they are awful people. Then she apologizes and acts fine. I really believe that she does get over it and really does feel bad but doesn't realize that treating people like that hurts and they don't get over it as fast.
My brother in law is at the end of his rope, nothing he does is ever good enough for her yet they have really good times. And then they have ugly times that he doesn't even understand. Like they will be fine and happy and just talking and driving around and pass a tattoo parlor or bar and she will suddenly get very angry with him and accuse him of all sorts of wild and irrational untrue things. I hurt for him because he loves her and wants to make the marriage work but it is also tearing him apart. He has lost nearly 20 pounds because of the stress. I want to share the info on BPD with him but she gets jealous if anyone else talks to him and I don't want to ruin my friendship with her. I love her and I truly believe that she is a beautiful, sweet, and kind person that is struggling with something very difficult but either doesn't realize it or doesn't want to believe it or both. I know she loves my BIL and wants the marriage to work and I know she would be devastated if my BIL left but at the same time I don't think he should have to live like that if things aren't going to change because it hurts him. I think him knowing about BPD might help him understand her actions though and understand her and that she really does love him and really doesn't mean to hurt him.
While they were working through the porn deal, she saw several counselors, therapist, and support groups but she has said they know nothing and do not realize that she is the victim and he is the cause. So, I have my doubts that she would even go see a therapist. She has opened up to me about her fears and anxiety and possible depression and I have suggested all the things that have helped me but she always comes back saying that no, she doesn't have a problem. My BIL is the problem, drs don't know anything, and taking pills is bad. The only thing I know what to do at this point is to be her friend and just listen and hope that helps. But I don't think it will save her marriage and I hurt seeing her and my BIL and nephew hurting when I think they could find help. Thanks for your responses and I appreciate and would love any more advice or guidance. Thanks!