I highly suggest renting a ECV for her, even if you think she may not need it every day. We average 7-10 miles a day at Disney. Places to sit and rest are few and few between a lot of the time. It's a LOT of walking and a LOT of standing. Since I injured my knee and back in a fall, I cannot do it anymore. I don't use any kind of mobility device at home anymore, but at Disney, absolutely. It is a totally different thing.
As others have said, a DAS doesn't apply to mobility issues at all, so the EVC is your best bet. There won't be a need to add your son's family to anything, and for almost every ride, you will be able to wait in line together. There are a few with alternate ECV entrances. Cast members are usually really good about directing you to the appropriate place. There are also a myriad of videos out there showing loading procedures for various rides.