Hi seabee,
I wanted to join
Wools and
Turkish and welcome you to the BPD Family.
It's clear that you care about your sister and that this has been hard on all involved, but IMO you have done the right thing taking care of you. The only people we can control is ourselves we can not make someone else do or be something they don't want to be and I think you've figured that out in relation to your sister.
So you took action to protect yourself and your girlfriend you set a boundary, a physical boundary and that is a good thing.
I know this change is hard for you my guess is that you have been taking care of your sister for a long time, that may even been your role in your FOO (Family of Origin) so letting go, setting this boundary feels strange or uncomfortable. But your sister is an adult and needs to learn to take care of herself, if you are like my SO he worried that his uBPDxw wouldn't be able to cope on her own, but she has she's gotten herself in some messes and still always had a roof over her head, food in her belly and clothes on her back.
In setting your boundary your sister is escalating things by bad mouthing you to other people. She's acting like the little kid in the grocery store... .
A little kid asks mom for candy, mom says no... .kid pouts. Little kid asks mom again for some candy, mom says no... .kid whines. Little kid asks mom again for some candy, mom says no... .kid has a full on melt down screaming tantrum. What happens if mom gives in and gets the candy? That little kid has just learned that having a screaming tantrum will get them what they want. What happens if mom doesn't give in? The kid learns that no means no and he gives up.
The screaming tantrum is the "Extinction Burst" that Wools mentioned.
In terms of the bad mouthing it sucks but try and let it go. The people that know you and care about you won't buy it, some people will buy it for now but over time will figure out the issue isn't you but your sister, and still others will buy her story in full (and do you really want those people in your life any way?).
Hang in there
Below are some links to more on boundaries that you might find helpful... .
https://bpdfamily.com/bpdresources/nk_a120.htmhttps://bpdfamily.com/content/values-and-boundarieshttps://bpdfamily.com/message_board/index.php?topic=61684.0https://bpdfamily.com/message_board/index.php?topic=167368.0In terms of the Therapist, she is your sister's Therapist and her choice so not much you can do there again that is up to her.
You might try and find Therapist of your own to work through some of the issues related to your sister, that professional support can be truly helpful and you always have us too!

This is a great place to get support, tools, ideas and sometimes just a place to vent.
Again welcome, I'm glad you've joined us,
Panda39