I'm a second wife to a man with an ex I'd rather not see. There are 2 children who were 11 and 8 when we got married. The ex will always see me as the "other woman" even though no sane person in the universe thinks that a 3rd party can break up a healthy marriage, and she left the marriage emotionally 3 years before my husband moved out. I could tell you stories... and she probably has a few herself.
Does your stepson have children? I can see that people with little kids just don't want to uproot them all the time for holidays or explain why Grandma can't be with them at the same time as Grandpa and Nana. People with kids want to stay put and have the child-free adults come to them, at least most of the time.
I don't know why you and she would resent each other - I'm sure there are plenty of stories and perhaps she is the most evil person in the world. But here's the bottom line: YOU are with her ex, and she is not. There shouldn't be a competition here, but even if there is, you've "won" and you have the good marriage.
When one of my stepdaughters got engaged, I was invited to a shower given by the mother's sister, with 60 people (yes, really), and I didn't know anyone there except the 2 women. I held my held high, put on a great outfit, and attended, being as charming as possible. When I held a shower for our friends, I gulped and invited the mother and even the same aunt. I made it the most fun I could and served the best food I could, making everyone feel comfortable. No one can believe I pulled these 2 events off, apparently, because most people wouldn't.
We don't do a lot of things together, but we certainly attended the wedding and took a backseat to my husband's ex, letting her shine. I forced (yes, forced) my husband to ask his ex-wife to dance, and I made sure the photographer got a few pictures. I told the bride that this was a wedding gift! When she had her children, we attended their christenings and took a backseat. We didn't seek attention, but we didn't hide in the background or act ashamed or like uninvited guests either. We made it as easy as possible for my stepdaughter. It was the right thing to do.
When my other stepdaughter went into a domestic violence shelter and her mother was wringing her hands and moaning, I'm the one who put together a "go bag" and a jar of inspirational sayings, bought her breakfast, filled her tank with gas and sent her off on her first steps toward safety and freedom. Her mother, who enabled this daughter's behavior/choices since she was 17 and modeled victimhood with men she was with both before and after my husband, is pretty much a mess in life. I'm the one who gets things done.
I suggest, for the sake of your marriage and your relationship with your stepson, that you hold your head high and show that you are the bigger person here. You have "her" husband, you have (I hope) a good marriage, and you have the sense of security and self-worth needed to get through it. This woman may be the most horrible person in the universe, or she may be a needy and useless wimp, but either way, she is no threat to you! If she goes low, you go high. And it will be in her son's house, and he will be in charge should she decide to do anything stupid. It won't be your problem. Go, smile, be a great stepmom, without being her best friend.
If you don't go every year, that's fine - you can spend time with other family members, just as many adult children spend one holiday with one spouse's family and the next year with the other spouse's family. But if it were me, I would not split up from my husband on a major holiday. My husband will have lunch with just one of his daughters, and that's fine. But a holiday or a birthday, that's for us together. We have plenty of times with just the kids at our house - my birthday, his birthday, Father's Day, and by and large we don't see them for every Thanksgiving or anything. This year, one daughter was with us and the other was with her mother. It's okay.
If there's something really horrible behind your story, that's something else again. We don't know your whole situation so of course you have to be the final decision maker. But if you are comparing yourself with her, don't.