I agree with Isca.
Go to the wedding so you don’t have to deal with the consequences of not going. Concentrate on their future relationship - not your past relationship with your mother.
Try doing an internet search on “wedding toasts” or even “Wedding toasts when you don’t like…”. Read through others speeches for ideas. Keep it relatively short.
If you can pick out a time in your life when you and your mother got along - even for one event, then try to incorporate that into it. Something like this:
"Mom, do you remember the time you took us to a baseball game? We couldn’t have been more than six or seven. We kept yelling at our team to score a touchdown - and you kept trying to tell us that they don’t do that in baseball.
Well, sis and I may not know a lot about baseball, but we know a good guy when we see one. Mom, you found good one!".
and then do the wish for a happy life together.
You haven’t dwelt on anything bad. You haven’t been untruthful. You’ve been gracious, and, you’ve satisfied what your mother expected of you.