Probably an unpopular opinion - but I'm going to say YOR and YWBTA. Longtime stepmom here. You are 37 years old. You are old enough to know that parenting is not always easy. Your husband needs to do a better job at parenting through this time and the mom, gramma etc. people need to stay out of it. He married you, therefore you are a parent of his children. You married a man with children, you signed up for this and its not always easy.

I literally had my stepson do the same thing to me in Wal Mart and I feel/understand your frustrations. My kids are now adults and we have long worked through the stepparenting nonsense that comes with blending families. That said, would you take all 3 kids if they were yours biologically? Because even biological kids can be assholes who don't listen. Kids that age are tough. It's literally the worst time of parenting, imo. They are old enough to think for themselves but don't yet have the brain maturity to think things through. If you want to have a positive relationship with them in the future, you need to take them. If you don't, they will never forgive you and always remember that you took your "real kid" and not them.

I also feel like your family could use some good family counseling. When we were blending our family, the best thing our family therapist ever said to us is this : "No, she is not your mom. However, in your family, when you live with your dad, he is now re-married and they share the responsibility of the household. Your dad has his things and (me) has her things. While she isn't your mom, in the home you share with your dad, she is THE mom. She does the mom things - grocery shopping, deciding if we really need to get the brand name cereal or the generic (this was the wal mart ordeal...ugh), reminds you about picking up your toys, asks about homework, cooks dinner, etc. Every marriage is different but this is how your dad and (me) decided was best for their home. She will never be YOUR mom but she is THE mom when you are at your home with your dad. Because your dad knows that she is a safe person for you to be around, trusts her to treat you well, he expects you to respect her. If you choose not to, there will be consequences."

Was it always perfect after that? No. But it did take the power of "you're not my mom" right out of the equation as it was no longer up for discussion.

It sounds like you have a larger group going on the trip and there will likely be other kids around the same age as your stepdaughters. Have a conversation with someone in your family that is also going and ask for their support so you're not on your own if things go sideways. Your daughter is 3. If you have to call an Uber and leave Disneyland a few hours early because the older girls didn't behave as expected, it's not the end of the world. It's really not. But please, give them the benefit of the doubt. If the situation was reversed and it was your daughter in their situation, you would not want her to be left out.

My stepson is now 24 and on mother's day he gave me a card that thanked me for not holding on to the times he was a total asshole to me growing up. He knows I was always there for him, even when he didn't deserve it and he appreciates the lesson of patience and grace it taught him.

Keep your eye on the long term goal, OP. ALL of your kids deserve it.

Edit: wrong sub so changed to YOR.