BPDFamily.com

It seems like nothing helps. She went to a psychiatrist and has been diagnosed but treatment makes her worse.  It gives her an excuse to lash out and focus on her problems. Now she is out of school and being homeschooled because school made her worse in many ways. The smaller her world the more healthy she seems but that her world is so tiny is also a problem. It sure puts more strain on the family. I feel so sorry for her brother who is a great kid but has to deal with so many weird situations. Has anyone else found that treatment had the opposite effect than intended?  Anyone have any advice for raising a young teen with BPD ?

Welcome!  We get it. 
~ The good news is she is in treatment.  Oftentimes it will get worse before it gets better. Therapy will challenge her beyond her comfort zone and her maladaptive coping skills and change is hard. 
~  Actually, you are spot on and don't realize it yet.  You have her in therapy ( check) , you are reaching out here ( check).   I will add read up on BPD , there is a "library " here that has book recommendations. 
~ Remember , you  and your son are as important as the BPD child.   Don't forget you need a support network as well while she goes through her healing.   Many people go to al- anon family groups, free and they help with detachment and keeping the focus on you .  There are online support groups  as well.   
~ Have patience because there is no quick fix for BPD. It is long, with twists and turns, so your self care is a must.
Keep writing here as you are able , you are amongst kin here.

What type of support and therapy have you tried ? Also is there meds involved? They have a great support group at whitbyshotes if u are in the durham area for parents with loved ones dealing with bpd