Realise that she sees this as a hold she has over you.
A thing she can use that makes you both uncomfortable - and that she enjoys prodding that pain.
Now decide - is she worth it? Is giving her the gift of your time and presence worth it? Really?
If not, your lives will be better off not having her in it.
If yes, you need to deal with her. Stamp on it. No eggshells, no anxiety.
Before you see her next, write to her.
Saying something like 'Looking forward to seeing you, with one proviso.
We've both noticed you've developed a habit of raking over a very old incident and bringing it up in conversation (usually after a drink?).
This causes us both distress.
We're sure you don't mean to do this - but unfortunately, that's the result of what you choose to do.
And the last few times you've done this have caused what should have been a fun and happy evening to become uncomfortable and upsetting to us both.
This needs to stop.
So next time we meet, let's consider that in the past, and the door closed.'
And if she does it again despite that? Get up and leave. She will have made her attitude to you both absolutely clear and made a conscious decision not to improve.