Goodness me, 17 years on and she is still harping on about it? She wasn’t even the wife! What a sanctimonious, bullying cow.
We’re none of us the same person we were seventeen years ago. We all do things we’re not proud of, or others may disapprove of, and those who think they never do/have are, frankly, delusional hypocrites. What matters is how we develop, change and make amends.
You may have been the OW 17 years ago, OP, but your SIL is a bullying, self-righteous, delusional, overly-harsh and unforgiving bitch right now, every day.
I would be starting to fight fire with fire.
‘Wow, SIL, you have an unusually strong capacity to be endlessly amused by the same repetitive ‘joke’, don’t you? I’ve never come across that trait in an adult before.’
‘Something really terrible must have happened to you, SIL, to make you so bitter and sanctimonious. Any time you want to talk I’m here for you.’
Or, if you no longer care about repercussions....
‘For the love of God, what the fuck is wrong with you that you feel the need to bully me over something I did - not even to you - 17 years ago. Fuck off away from me until you finally manage to grow up’.