If you can't take a child that age to Disney, where can you take them? It is a perfect family vacation for kids of all ages. Take her with you, 6 months is a great age to go. They aren't eating constantly anymore, but don't need to get down and be mobile all the time yet either.
I am with some other PP's in the boat of "I couldn't ever leave one of my kids behind" My kids get one on one time occasionally at home with either DH or I, they don't need it on a family vacation. Then again, I might not be the best to take advice from because I seem to be the minority amongst most people I know. I spend all my time with my kids and I don't mind it one bit. People don't usually believe me, but it's true!
Take her and make some memories with the entire family. After all, the older kids had your attention more than she ever will being the 3rd! Have a great trip, and I know what you said about your mom, but the reasons you listed would also be reasons I wouldn't want to leave her behind with her, Just MHO!
Andrea