How to handle bm being invited to family gatherings?

Hi all, I have a problem which is not directly towards my step kids (they are lovely), but my own inlaw family. They constantly invite bm, and her family as well, to family gatherings. Step kids are in their 30s, so it's not like they need her there. It would be one thing if bm had a civil divorce from my husband, but it wasn't, and all the contentious behavior was on her part. The divorce itself was her fault. She treated my husband very badly. Because of this inviting of her, on top of other mistreatments, my husband doesn't really speak to his family, but will attend family gatherings if he's available. An important gathering has come up, and he's not available to go. I am being pressured to go with our kids, and I don't feel comfortable to go. There is virtually no one going that I could sit with, and my step kids will most likely stit with their mom and her family. What do I do?