"Mom, we've been invited to spend time with my stetp-sister and her family. We can make a plan for you to come visit us another time." This will be your go-to line. I would not engage in discussions about this because if you do, she'll get the idea that it's still up for debate if she can just persuade you or otherwise wear you down.
At some point, if she persists, you'll have to be prepared to say a very firm, nonnegotiable "NO" and then refuse to engage in any further conversation about it.
It may sound cold to some, but that is how you have to deal with demanding, over-bearing, self-absorbed people who want to control you. You have to have greater resolve and make "NO" your final answer. Then, be done with it. Shed the guilt and realize it is not your place to be your mom's sole source of entertainment and socialization in life.
Even if your mom were the greatest gal on earth (and clearly, she's not), you are under no obligation to bring her with you on every family vacation.
It's not your responsibility, nor is it possible to make other people happy. We are all responsible for our own happiness. It's sad for people like this, who seem to have no insight into why others don't want to be around them, but it's not your responsibility to fix that. Whether she ever chooses to accept this is solely on her.
Good luck, stay strong, and enjoy a peaceful spring break vacation!