Wow! My sympathies to you. What a mess.

Listen, you are taking on too much responsibility for your sister's behavior and her relationship with Mom. I don't think there is any "diplomatic way to discuss this" with her, because there isn't a thing you could say to her that she would hear. She has made up her mind to try to hide her mom and excuses her cruelty because it is Her Wedding -- believe me, you can't fight that kind of tsunami, and it isn't your responsibility to do it. She will ruin or strenghthen her family relationships as she will, and if you try to interfere, no matter how good your intentions, everyone will end up mad at YOU. It just always seems to happen. It's really hard to stay out of it, but it's worth it.

Instead, I would focus on what you can do directly to help your MOM feel better. I know you will be busy, but maybe you can keep your eyes open for opportunities to drive her, spend time with her, keep her up to date on fun things about the wedding (if that would make her feel good, not bad), etc. I would definitely stay away from language like "zealot," "anorexic," "plastic surgery addict," etc. The last thing your family needs is more drama and bile.

Good luck to all of you.

same thoughts here. you and your mom should use this time wisely. your sister will have regrets for her terrible actions - maybe not now, but she will.

I agree with both of the above. One thing that comes to mind is the rehearsal and rehearsal dinner. Will your mother be invited to either? If not, that will really add to the hurt.

Clearly there is some past unresolved issue that is causing your sister to act in such a hostile manner toward your mother, but that is your sister's issue. Her new in-laws are bound to see how she treats her mother and that may cause concern on their part. If she has a good relationship with them, perhaps they can open her eyes to what she is doing.

She will ruin or strenghthen her family relationships as she will

I agree with this very much. I also agree that she will get the family relationships she DESERVES.

And, the rest of your family has a right to understand exactly how hurtful your sister is willing to be. They have the RIGHT to base their relationship with her on undiluted, factual information.

If your sister is willing to hurt your mother's feelings, your mom needs to know that. Then she can adjust her expectations, and perhaps save herself some acute pain in the long run.

I'm sorry for your mom, and I hope you can be a source of love and unconditional acceptance for her.

My guess is your Mom may have an idea this is going to happen (speaking as someone whose X has a plastic wife). Her flying in only a few days before is somewhat of a tip off. She may be planning on taking the high road, and showing up and just watching the show. You might try impressing on your sister that it will reflect very very poorly on her if your mother is not treated as MOB, front seat, etc, etc. Does your mother have a brother or other male relative for escort? Do you have time to spend with your mother? There is no need for her to meet future SIL, as your sister has decided she is not looking for her mothers advice. In a perfect world, they would have met before, but that train has already come and gone.

Thank you all for your wise comments. Yes, it is our mother who is taking the high road, she says she is not hurt or offended, and that she understands that logistically it is difficult for them to meet. I'm the one who seems to be the most outraged. As you suggested, my partner and I plan to escort Mom around, going with her from place to place and making sure she's having a good time. We also have a brother who may be some help. Some of her best friends will be flying in for the wedding too, so she won't be alone. It is just heartbreaking though to see my sister behaving this way, I never would have expected it. But I guess these sorts of events really do show people's true colors. Thank you for your advice.

Your mom is lucky to have you. Just remember if she can hold her head up, so can you.

Two events that seem to bring out the worst in a family are weddings and funerals. Try to do whatever you feel is the right thing toward your mom and kind of cruise below the radar if you can. It's really between your sister and your mom.

Your sister may also be under pressure from SM and your dad (who himself may be under pressure from SM) that they are paying for wedding, and SM is to be "respected" and that if she wants a big wedding, she better toe their line.

In any event, your mom seems likes she is going to be OK.

oh, the problems we have with outlaws...personally i liked my one dil's bio-mom more than the step...they (dad and wife) were so uptight, wouldn't come to the rehersal dinner we hosted, cause "she" would be there...

so enjoy your mother, and go ahead with being in the wedding party...class doesn't mean cash...