Well, she was already planning on visiting us at home. The problem is, we don't want to be home that weekend. And she has the money to visit us. Money isn't an issue.
So just continue your plans. There is only ONE sentence for you to say from here on out when this topic comes up. (Cut and paste!!)
"Sorry, Mom, we're not going to be home that weekend. Let us know if there's another time you'd like to visit."
No matter what she says, or whines about not seeing the children. "Sorry, mom, we're not going to be home that weekend. Let us know if there's another time you'd like to visit."
Don't get into what her other plans are--don't even ask, don't put out so much effort in trying to arrange for her to visit. She can come when she wants. You'll travel when you can afford it (in terms of time or money), and that's it. There is no guilt--these are facts, and guilt has nothing to do with it.
You might send her an email and say, "These are the times we have off and will be at home. You of course can visit when you'd like, even if we're working or in school--it might be more boring, but you'd see us in our 'real lives.' "
Don't get into how much it means, etc. Stick strictly to the logistics. And when she tries to pour the guilt on, suddenly the cat's on fire or something--"Sorry, Mom, gotta go. Send us an email when you've decided when you can come."