
I really hope you don't mind me posting here. I am from the board of parents with (adult) children with BPD. The only reason that I am reading posts here is to learn what my children may be coping with so that I can be empathetic towards their situation.
I do believe my ex (who is the father of both my children) has BPD, but I am not into ex-bashing and I say this with a heavy heart since I didn't realize what it was before our marriage ended. Even though I am in an amazing relationship now with someone much more suited to myself it hurts to see what has happened to him since I have been gone. My ex is a good man, honest as the day is long but he does suffer so many BPD traits that I now recognize since learning about BPD. Too much to say not related to this post.
What I do want to say to you, is that I have seen this play out between my daughter and my son. The difference is that they did become pitted against each other. Not as much because of what dad said about them but more to do with how he treats them differently.
This is what I feel that I need to say to you based on my experience with him and my son:
It is SO awesome that you and your sister recognize this and are strong together.
At the same time it is equally important that your mother doesn't see you as a team against her.
As long as she feels like you are a team against her she will try to split up the team.
How can you avoid this and stay strong together?
How about this:
Incorporate the SET (support, empathy, truth) method when she brings this up, but do not otherwise engage. After the SET response be firm that you will not engage and if the subject cannot be changed then end the conversation.
Do NOT j.a.d.e (justify, argue, defend, explain). What I mean is don't defend your sister, explain her behaviour etc. Why? Because you need to show you are not a team against her.
Within the SET method of communication let mom know gently that you understand she has issues with sister, and when you get to the truth part let mom also know gently that sister doesn't talk that way about mother. Or, if that isn't true then let mom know that if sister wanted to talk to you about mother that way you will not engage either.
Let mom know you won't engage either way. The point is to make sure she gets constant reminders that her relationship with you is not threatened by your relationship with your sister (real or imagined).
Stay firm.
Rinse and repeat! LOL Simply meaning that you are going to have to do this often.
I imagine sister and you might choose to let go of the hurt and practice radical acceptance. Know it's mom's BPD talking. You can always call her and say "tag - I'm it"! to let your sister know you just went through this with mom so it's in her ballpark now.
Try to make light of it. I'm not saying to laugh at your mom but maybe you and your sister could take the stress off of it by having a little joke of your own when you pass the ball.
In the end, chances are that one day your mom will be gone and it will be your sister and you left. Hopefully you can look back and say you did the best you could.
Thanks for hearing me out. I don't know if I am way out of line here since I am not speaking from a child with parent experience. Just had the urge to reach out.
R