Anxious about attending this wedding

Families!? Step-families!? They can be a nightmare can't they? I can only suggest that you try and put twenty years ago behind you. Not easy I know!! (been there and got the t-shirt!) Have you seen this woman at all in the interim? If so - can I assume that little was said or have you literally not seen hide nor hair of this woman? If the latter applies - can you arrange a meeting even briefly so that the wedding is not when you first set eyes on each other? As for the wedding itself - it is possible to greet each other and then surreptitiously avoid crossing paths for the rest of the wedding.
Has your step-son any feelings or input about this potential issue? I can say from experience that often we build these things up in our imagination and ultimately it ends up being nothing at all in the cold light of day and as this is a new relationship then the past really should not have any effect on the present.
I understand your issues and even now any invites to family occasions will freak me out momentarily when I know my husbands' ex-wife will be there. After 21 years she and I now pretty much greet each other and say farewell that is it!!

20 years is a long time, you’ll both have changed over the years.

Be pleasant and courteous when you meet her, no referral to the previous trouble. Hopefully she will respond in a similar manner. If she doesn’t, well, that is her problem, don’t enter into it, just let it go.

It’s only one day after all.