The other issue I have is my siblings were able to get through their weddings and now have an ok relationship with her but this just 'feels' different.
I think they pay for this ok relationship in ways that are hard to see.
Were you the scapegoat in the family?
In pathologic families, it is often the scapegoat who goes to therapy and learns to do things in different, healthier ways. Often this involves introspection and an ability to experience vulnerability and intimacy and spontaneity, which are qualities that can be quite threatening to dysfunctional families. Families with BPD patterns tend to use rigid roles to function and getting out of line elicits swift punishment by the rest of the family in an effort to stabilize and return to dysfunction.
People with untreated BPD tend to have no boundaries. We need boundaries to help us define who we are. Getting along with someone who has no boundaries often involves some kind of sacrifice that can have an impact on other parts of our lives and even how we feel about ourselves.
There is a cost to estrangement but if it comes at the expense of having a healthy boundary then most likely it is worth it. Giving into a bully in exchange for the illusion of peace only empowers bullies more. It's inevitable that more of the same behavior will follow.