I would simply tell mom “I’m out of the will then” and then drop all contact with her. Toxicity has no part of my life

Listen. They want you to go to the wedding and play happy families so that Great-Auntie Deborah can look at that and think “Oh well I guess OP is fine with her sister marrying her ex-husband since she’s here, so I’m fine with it as well”. If you do not show up at the wedding, Great-Auntie Deborah and her clan will recognize this for the betrayal it is and judge your mother and her golden child for it, as they rightly should. You do not need them. They need you. And screw that. Show off your spine and tell them to go pound sand, because no amount of money can buy self-respect.

If I was you OP, I would let HELL rain down on your sister's wedding. Show up dress to the 9's and look even better than the bride 😁 then give a lengthy speech about how your sister and ex-husband met 🙄 and end it with telling your mom to go **** off and call your mom out, how dare she pull BS like that. Is your mother delusional, nobody in the right mind would ask their daughter to show up to the wedding of their other daughter who's marrying the cheating ex!! If those were my daughters I'd be disowning the one getting married.

I’d do it and wear a big white t-shirt over the dress with my feelings written on the shirt in big red letters. Then tell mom to **** off

Ok, you were the "practice wife", so what? No need to be so bitter. Marriage is hard and people rarely get it right the first time. Put on your game face at the wedding and get back in mom's will. Then, hire a hit man to expedite your payments. With all that cash, you'll be able to afford your "real husband." You gotta play the long game and not get your mushy "feelings" get in the way.

I wouldn't go. or agree to go and never show up and tell Mom to **** herself with a pine cone

Don't go. Keep your self-respect and dignity and your mom and sister can talk to the hand.

The audacity for your mom to call you selfish after what your sister did to you and is now marrying your ex, we know who the selfish one is. You deserve much more than that, especially from your mother. Do what feels right to you, don't let your mom give you an ultimatum especially on that level of selfishness. Because she could also throw it in your face later that you were your sisters maid of honor so you could be in the will. Have your self respect, **** what anyone else says or thinks.

I despise people who try to use inheritance as a means of controlling people - almost as much as I have disdain for people hanging around waiting to collect an inheritance… While it’s not impossible that Mom might actually have assets worth inheriting, it’s also possible there aren’t actually any great value in a supposed inheritance… Personally I would refuse to be controlled by promises or hints of a possible inheritance. Once you start down that path it never ends…

This is why I'm glad that, where I live, you cannot disinherit your biological (or adopted) children, to prevent manipulation like this. Also note that she only said you had to show up and be the maid of honour, not that you have to behave. Then again she'd probably find some other situation to threaten you with down the line, so just tell them to stuff it, let the rest of the family know about the despicable threat (or they'll be treated to your mother's "truth") and cut her out. Just because she's blood doesn't mean she's good for you.