Did you ever learn a big secret about BPD parent?

[sanemom]

We are going down a scary road right now.  DH was married to a woman with BPD for 4 years, but she left him with three young children to raise for ten years.  She did her standard visitation (missed a lot), but when we got married, she filed for custody suddenly.  BPD mom is a waif and it is clear that the children (now teens) feel the need to protect her.  I don't think she is overtly cruel to the kids, but she definitely emotionally manipulates them.  The oldest (16) is the golden child and moved in with her BPD mom a couple of years ago.  I don't think she is happy there, but she makes the best of it, and I don't think she feels she can leave.  BPD mom told DH that if DSD moved back in with us, BPD mom would die…I am sure she has told DSD the same.

We have been in a legal battle with some pretty crooked people.  DH has fought to get counseling with his now-alienated DD for 2 1/2 years, and the GAL has been blocking it right and left through very unethical means.  We just found out that the GAL has been blocking it because he was told by BPD mom that DH sexually abused DSD, and he just believed it.  No outcry from DSD.  No investigation.  Nothing.  The accusation is completely false.

BPD mom has told many people that accusation to justify why she has separated DSD from her siblings. 

It has now come to a head, and the court appointed therapist said that she is going to have to let DSD know about these accusations from BPD mom.  She didn't want to bc it would devastate DSD.  DSD truly does not know how cruel BPD mom can be to others because mom hides it from her kids.  She keeps the cruelty under wraps by emailing it and texting it--the kids seem very unaware.

I am a little worried that DSD will feel the need to protect BPD mom so much that she will suddenly claim something happened.  I don't really think she would throw her dad under the bus like that by lying, but who knows how she will feel.  She seems so enmeshed with her mom, but, at the same time, I think she is starting to realize that her mom has lied to her about some things and she makes lots of broken promises to her.  DH has been the only stable factor in her life, but BPD mom has lied to her about him, too.

Any ideas of what to expect when a teen finds out such a big harsh reality?  She has been in such denial that maybe she will do that again... .