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What can my 16 and 18 year old kids say to my sister when she is alone with them and tells them awful, untrue things about me? Example: she's certain I'm going to steal from them or kick them out of the house, says I abused our parent, etc.
   Background: mom had BPD. I was preferred child. Sister was not. Sister has always inserted herself between me and anyone who cares about me.
   Now that kids are young adults she is buying them with expensive gifts/trips and they are asking me about the horrible things she says about me. I don't want to triangulate by discussing this with her. I want to empower the kids with a phrase they can use when she trashes me to them.
   What do you think of "She loves you and is scared you will go to college and leave her. When she says bad things about me just tell her 'I love you and you don't have to tell me bad things about my parents for me to love you. Please stop doing it."
   Thoughts?

Ugh. Triangulating kids into these dynamics is the worst.  

With my son (BPD dad), I used validating questions, which helped refocus things on the last thing your sister cares about, which is their feelings.

Them: "Aunt BPD says you have a horn growing out of the base of your spine."

You: "Geez. That's a new one. How did you feel when she say that?"

Them: "Idk. Weird, I guess. She says a lot of stuff about you."

You: "Yeah. I guess I'm wondering how that affects you, what it feels like."

Them: "Well pretty bad. Do you have a horn?"

You: "Gee. I guess I can't let go the idea that it hurts to be put in the middle like that?"

It can be defang the accusations because you don't lower yourself to engage in the silliness, and it derails her agenda, which is to win at the relationship with your kids (putting you in a one-down position, where she needs you to be to feel better).

An expert on parental alienation (a version of what you're experiencing) also encourages using humor if you can pull that off.

You: "Ha haha, oh that one. She's always saying that kind of stuff."

If your kids have already said they don't like it, they could try shrugging, too. My stepkids do this with their mom and it seems to work to some extent.

I think the key is that you want your kids to know that you care about their feelings first, then yours. Unless you genuinely feel this is about you. For my son, having a BPD dad, he was acutely aware of who put his feelings first and was always scanning for that signal. He had extraordinary abilities to tell when I was concerned about myself versus him -- it is very natural to want to defend ourselves and our reputation, but for them, they are being put in the middle of a dynamic and want the adults to show them how to get out of that triangle. We can't do that if we are prioritizing the defense of ourselves. That has to come in a distant second or third.