I'm sure this is a common thing among spouses of those with BPD, but I really can't take it anymore. She is constantly bashing every little fault or mistake they make. I have been guilty of stepping in to defend, but that obviously doesn't go well. How do you just sit and listen to lies and accusations about those you love?
Those lies to you may be perceptions to her. She may truly see them that way. Defending them means, to her, that you think she is wrong and they are right. My wife accuses my mom of a lot and hates her. A lot or all of it is her perception of my mom's actions. She considers what my mom does to be an attack on her when my mom is thinking nothing of it.
I used to ask her if my mom meant something else since I never witness the event. What drives me crazy is when she gets angry at my mom and sees me as the proxy. I do not have to defend my mom to bring it upon myself.
How do I listen? I keep quiet, listen, tell her that I understand that she is upset and attempt to shift the discussion in subtle ways. I do that with my S3 and have started using it on her. It is sad that it works. I cannot recall an example to give you, but it can work. It at least can get them past the rage period where you can talk to them afterwards more rationally.
Contrived example (I think better in the actual situation):
Wife: Your mom did X. I hate her.
Me: I understand you are upset/angry. What happened exactly?
Wife: Your mom did X when I wanted to do Y.
Me: That is odd that she did not do Y.
Wife: I know! She does it to spite me.
Me: Hmm. I will see what she thinks about doing Y next time.
Wife: She should do Y!
Me: X is nice, but Y may have been better. X is fun in the Z situation.
... .
I try to make X not as bad yet let her complain about it. I try not to say Y is bad in that situation or X can be anything better than Y. Sometimes drawing it out can calm her.
What I would like to happen:

Wife: Your mom did X. I hate her.
Me: Who cares? Stop whining! Bring me a beer!
Wife: Yes, dear. You are right. Here is your beer.
NOTES:
- She tells our children not to whine when they are sad about something. I want to tell her the same because I feel they are whining less than her.
- I really do not like beer. I just wanted to sound chauvinistic.
I need more humor in my life.