One: If you are traveling and staying in the Disney area, food is going to be an issue for your MIL unless you have a suite and she orders in diet frozen dinners or something similar. Is there anyplace in Disneyland that serves low cal meals? Maybe at CA, but unless your MIL plans to leave the parks to eat, it will be a challenge.
Has she taken this into account and made plans for eating? Honestly, eating at the buffets, etc, won't be any less healthy. In fact, it may be her best bet for getting lean meats and salads.
Two: You might want to point out to her that while Disney is for families, the focus...or at least your focus...is on the kids. If you have cell phones or walkie-talkies, she can go her way and do her own thing and it won't be a problem to catch up with each other after meals. Or, if she wouldn't have to eat at all....
I'll be honest, this is one thing that drives me crazy. People assume that fat people have food at the center of their lives. I've found it to be that people who fear being fat are the ones who put food at the center of their lives!
Has she taken this into account and made plans for eating? Honestly, eating at the buffets, etc, won't be any less healthy. In fact, it may be her best bet for getting lean meats and salads.
Two: You might want to point out to her that while Disney is for families, the focus...or at least your focus...is on the kids. If you have cell phones or walkie-talkies, she can go her way and do her own thing and it won't be a problem to catch up with each other after meals. Or, if she wouldn't have to eat at all....
I'll be honest, this is one thing that drives me crazy. People assume that fat people have food at the center of their lives. I've found it to be that people who fear being fat are the ones who put food at the center of their lives!

Thats to bad! People who dont totally LOVE Disney have a hard time understanding why anyone would spend that much $$ on Great seats!
My MIL and FIL, think we are out of our minds "waisting our money" on takeing the kids to a "NO Holds Bar" Disney trip. When they do show up...this was the first year they did not...they Never want to eat where we have made plans.
. We will be heading back for 3 night at the PPH on Aug 1st. and we have a family reunion planned. My hubbys side of the family. Most all the adults with out kids are staying 1 night. Too much Disney for them..Hee! Heee! 
including her MIL, she did say..I think..that she was paying for other things for her MIL on this trip. Yes its costly to do this, if the MIL cant she can still be sweet about it and not tell her shes crazy.
Its a one time thing for the MIL. And if shes only paying for herself...this is their vacation that they were nice enough to include her in. And if memory serves...the MIL invited herself.