I, too, am glad to hear that your husband is standing up against her destructive attitudes and behavior. Adult tantrums are very difficult to be a part of but you showing Christ's love to her and her children can be the good that comes out of it. Trust that at some point this behavior will end, the problem is she can choose to end it by destroying the family or she can choose to end it by following her husband and build an even better family for herself and her children. You be the best friend you can be, within good boundaries, and pray for your husband to be a good spiritual leader and husband to her.
As for Thanksgiving, that is a hard thing. I have known several plural families who do this regarding EVERYTHING in their life because of various reasons. I say this because, even though it isn't what you would like, it is something that can be done and certainly for a short period of time. I encourage you to view it as an opportunity to build a relationship with the extended family on a none threatening basis. This is your chance to show them who you are! Also, it can be a chance for you to see how your husband and SW celebrate Thanksgiving. Pay attention to what is important to them and how they show their love to their family. This will help you understand how you can help in this celebration in the years to come.
As for Thanksgiving, that is a hard thing. I have known several plural families who do this regarding EVERYTHING in their life because of various reasons. I say this because, even though it isn't what you would like, it is something that can be done and certainly for a short period of time. I encourage you to view it as an opportunity to build a relationship with the extended family on a none threatening basis. This is your chance to show them who you are! Also, it can be a chance for you to see how your husband and SW celebrate Thanksgiving. Pay attention to what is important to them and how they show their love to their family. This will help you understand how you can help in this celebration in the years to come.