Okay, so you started with one trip for yourself and your GD. Then, because you were afraid you might hurt the feelings of a grown adult, you invited your son (GD's dad) on that trip. I'm assuming you then invited you other son and DIL, who apparently said, "no that time isn't good for us", so you're taking YET ANOTHER trip for them, at their convenience.
In order to get some kind of peace and quiet, you've decided to take two additional trips on your own. And your DIL invited your ex to go with you on your own trip. And your mother wants you to take her on the other trip in order to wait hand and foot on her.
Stand up for yourself. Let DIL nicely know that you are no longer married to your ex for a reason. Let your both of your sons and DIL know, "This is the time I am able to take everyone, if you can't come, then I will be disappointed." You're planning 11 months in advance, if they want to come, they will.
Also, let everyone know EXACTLY what you plan to pay for. I wouldn't pay for food or spending money for grown adults. And I would ONLY pay for all of the trips for my grown kids if it was something I could easily afford. Even then, I would let the adults know that the trip is their Christmas present -- no need to get anything else.
Also, let everyone know that your other trips are for you. Don't give them your flight info or hotel plans (until and unless necessary) and no one can plan to go with you.
As for your Mom, let her know that your trips are for you and that while you would love to spend time with her, you don't think that you would be able to provide all of the support she would need at WDW. Then discuss something local the two of you might do together -- a nice show and lunch? Maybe plan a few different things.
Don't be afraid to be rude. Be polite, but firm. And don't let others make your decisions for you. If someone wants to be offended by the fact that you are finally standing up for yourself, then let them be offended. You can't be held responsible for everyone else's happiness.
Best of Luck!
EthansMom