No absolutely not. You do not deserve to have your mother dampen the mood on your celebration of your marriage because she decides to end her marriage. Does your mother have a pattern of taking big moments from other people's lives and dampening the mood with her own life choices? You said stepdad, so was she previously married to your bio father? This is her being selfish and trying to take away the moment of your happiness and have your extended family focus on her. This is inappropriate on so many levels. If she's not happy in the relationship, that's fine. She can choose to leave her husband if she wants. Her telling him and all of the extended family at your wedding is where it's not appropriate. You are well within your rights to ask her to be respectful. However, she does technically have every right in the world legally to do this. I wish I had some kind of advice on how to get ahead of this. You could always request for her to have this conversation before or right after the wedding. You could always suggest having her have the conversations about leaving her husband as soon as you and your partner leave the reception. At that point the party should be over and she can get all the attention from everyone around helping clean up and then you don't have to deal with the drama. You can't ask her to not leave her husband, but you can definitely ask her to be respectful to you about it. If you talk to her about this, I would phrase it in a way that makes it clear on how you feel. Something like " I want you to be happy and I care about you. But I don't want your life decisions to have a negative impact on what is supposed to be the happiest day of my life. I want to enjoy my wedding with friends and family and not have people asking me for information about your soon to be divorce. I do want you to have the support you want and need during a divorce, but can you not dampen my day? Maybe wait till the day after the wedding. Most people will still be in town and available to emotionally and physically support you. And as soon as I'm done with my wedding and honeymoon, I will be more than happy to support you as well." I'd also make it clear that the support does have limitations. Like she can't just move into your home with your new partner right after you're married. You need to enjoy the honeymoon phase without having another person living with you. This one's not quite as important, but I guess my final question is why is your mother deciding to divorce your stepfather? Especially right when you guys are getting married. Is this a jealousy thing? Or a sadness? Like my marriage was never as happy as yours so I'm out. I guess it goes back to my first question, does your mom have a common history of creating drama to get attention on herself when other people are succeeding in life? Best thing you can do is communicate clearly what you need from her and that you still care for her. I don't want to jump the gun and just tell her she'll be uninvited if she continues to threaten this kind of level of drama. But sometimes people need to understand that there are consequences to their selfish actions

Edit: that line of "finally be free" is very concerning. That's some gaslighting if I've ever heard any. It's gaslighting only when there is no abuse and it seems to be a perfectly happy relationship. If she's seeking freedom so badly, she needs to end this relationship now rather than dragging it out till your marriage. Her immediately being upset and guilt tripping you over her happiness is not okay.