You know what?
I would aim to actually do something really nice for a change.
I would tell her about her dh and say that you thought it only fair that she knows before someone else tells her. That is, providing you have it on very good authority that this has happened.
Why do you think she has been so mean to you in the past? Is she jealous of you? Has she had unhappy relationships before? You say that her dh is a serial cheat, so maybe she feels vulnerable and desperately unhappy because of this - sees your loving relationship, which makes her feel even more inadequate and is insanely jealous.
I would feel sorry for her. She has children with her dh, everyone probably knows about his indiscretions, everyone probably thinks she's a fool, she will be embarrassed by his behaviour and maybe in marrying him she is trying to get him to commit or to redeem herself in some way?
She sounds like a very unhappy person who has no real friends or self respect.
How flabberghasted would she be if you arranged to meet up for coffee and had a heart to heart with her? I am betting that her hard exterior would crumble and you would see the person underneath that thick skin of hers.
We all have our vulnerabilities, we are not born mean but are made that way. Underneath it all, we are all human and we all hurt.
Do something lovely and nice. Be a friend to her. If she rejects you then leave it at that. You will have tried.
Forget what she has done in the past. She only spoiled your wedding because you let her. I'm sure you have many more brilliant and happy memories of your wedding so why dwell on this one?
My SIL was horrible to me at my wedding too. I have always tried to get on with her even when other family members didn't. I sent her and her sons birthday cards and presents. I never knew that I had been spelling her surname wrong (she wasn't married to my brother at this point) until she came marching over with the place name I had made and demanded I change it. Then, her teenage son demanded that I buy him a drink because I had sent him a birthday card without money in it but a promise to buy him a drink next time I saw him. I was in my wedding dress, I had no money on me and he stood there whilst I asked another guest if they could please lend me money to buy him a drink.
I don't dwell on that though, that would be giving her the victory of having spoiled my wedding day. She didn't. I still send them all birthday cards though I never receive any back, not even at Christmas.
I am not like her, I have a heart. If she confided in me tomorrow (I know her and my brother have split up now) I would listen. I wouldn't bear grudges. What would be the point? Why hold bitter feelings? They don't make you feel good, they just make you feel angry. Life is too damn short to let anyone make you feel negative.
I would be concerned about how she would feel if she found out about his latest indiscretion. That is why I would invite her out for coffee and with all the best will in the world I would tell her, stressing how sorry I was and asking if there was anything I could do.
She might not thank you for it, but I wouldn't want her to get married to a shitbag and I'd want her to know that she did have at least one friend she could talk to, even if she didn't want to know. Because I'll bet she doesn't have that.
The way to crack open meanness is through kindness.