You don't want to start a precedent for everyone going on every vacation with you, so do some overall thinking ahead of time of what you see your summer trips (or other time of year trips) looking like. Also take into account people's likes/dislikes, mobility issues, food choices and more. But if you take them ALL with you this time, what will you do the next time, and the time after that? Be careful about what cans of worms you open up.
There's no nice way to say to your MIL that you've already planned a vacation with your parents. So I'd start out by saying you're thinking of a trip with MIL next _____ (choose a month or season) and wonder if she'd like to do X with you. For example, a road trip to a place where you rent a house is not all that expensive - we used to take my mother on some of our trips to Cape Cod (1-2 hours away from here), and we just rented a house with an extra bedroom. We cooked meals in the house, made sandwiches for the beach, and so on, so a lot of the daily expenses weren't much more than we'd have had at home. It didn't cost any extra to put her in the car with us either. It worked because my mother likes the beach and doing things like mini-golf, and also enjoys spending time on her own with a book. So we could go do an activity she didn't enjoy (e.g. go-karts) and she was fine about not going along. She also was thrilled to just observe her grandson doing things she didn't choose to do personally. She usually took out all out to dinner one night, maybe two, as her contribution. So see if you can plan something like that with her, or at least open the discussions with her about what she would enjoy.
If the trip with your parents is something she would love but cannot afford, you'll have to look at perhaps paying for her on another trip. But it depends on what would hurt her pride and what wouldn't.
If you're going on some expensive family vacation involving airfare and hotels and restaurants and amusement park admissions, there's no real nice way to tell her she's not invited because she can't afford it. So DO have something else lined up to at least offer her. And have your husband do the asking.