First and foremost, if I am ignorant in any way, please tell me how I may understand better.  And sorry about the no flair, I'm on mobile.

Some very important people in my life whom I love very much struggle with BPD.  It ruined the last relationship I was in, and I will not let that happen to my relationship with my sister or my best friend, both who I would absolutely die for.

I have an interesting relationship with both of them.  No matter what happens, I can't stop them from feeling like I'd abandon them or talk about them behind their back, and they lash out before anyone can hurt or get close to them.  What can I do in these situations to show my solidarity and dedication to these people I love?  Is there anything I can do to help reassure them in AND out of an argument like this?  What are some love languages for those with borderline?

Thank you in advance.