One of my boundaries with BPD mother is that I don't discuss other family members' personal business with her. She often triangulates- calls me to vent about a sibling. My response to her is a polite " this is between you and sibling mom, I don't want to be involved in this".
It takes a while for this boundary to set in. You don't have to say anything about the boundary to them- ( that would cause a reaction)- you just have to do it. Each time they start to talk about a person- your response is the same " That's between you and them... ."
Eventually they don't get the result they want from you- the triangulating. They may still do it with others who are willing to participate in this with them, but so long as you aren't participating in it, you aren't

I do talk to sibs about mom. Part of this is that she isn't consistent in what she tells us- tends to lie. Because she is elderly and alone and we are next of kin, we cross check facts to know how she is doing. We also talk to each other for moral support. But as far as I know, we don't participate in triangulating with her "against" each other.