<human>: Condense the text into a few key points

She was my childhood best friend for a long time, but now things have gotten weird. 

So my cousin was diagnosed with bi-polar a while ago. Fine. She has struggled with eating disorders for the last 10+ years, through which I have tried to support her. In the last couple years, she has been diagnosed with Borderline Personality Disorder (and she's oddly in love with this middle-age female therapist who diagnosed her, even though she is normally straight).

She met my current SO - flirted with him (?) and tried to embarrass me by bringing up my first sexual encounters (unwarranted slut-shaming) and trying to make it look like I helped her be the "other woman" in a different situation (no, I didn't). She has ruined 2 family weddings so far with her dramatics. 

CURRENT ISSUE: I just got a call from my aunt that my cousin says I'm breaking up with my SO. I'm not, and she would have no reason to say so. We are all about to attend my cousin's sibling's wedding and I'm terrified of what could go down.

Do I explain this to my SO before the next time he sees her? I don't want to look like a drama inventor, and I trust my SO to understand situations quickly and empathetically, but I don't want to put him in a weird position.
<bot>: my mentally-ill cousin is weird about my current relationship and told people we're breaking up when we're not. do I warn him, or do I wait to see what happens?