Woh, that transportation post....

I had forgotten how it felt to travel (by car) with two littles. All the planning and packing and breaks for diapering and feeding..... and then, still, not knowing how it's all going to work out in the end...

You are a rockin, brave momma!
I'm sending you lots and lots of pixie dust that the boys enjoy the ride and don't get fussy and you have a painless drive to the World!
Hmmm, the stroller issue. That's a tough one. I think I'd let your sister have her way. Honestly, the worst that could happen is her little guys poop out and she has to rent a Disney stroller. If it happens every day, well, tough lesson learned for your sis.
But, she may only need one on a day or two. My son walked the parks for 10 days when he was newly five. A little older, but not a huge difference. He did great. Only one night did he fall asleep on dad while we were waiting for the fireworks and couldn't wake back up - so dad carried him to the buses, for the (standing) ride back to Pop Century, and then back to our room.
Granted, that would be a lot tougher with two kids, but with four/five adults, and if it's only a day or two that they get so exhausted, you could make it work. And one double stroller will make navigating the crowds a
lot easier the rest of the time. I think it's worth a shot. Just make sure she's open to renting in a pinch. There's no point in making everyone miserable over $30 (or whatever a double stroller is nowadays

).
I can't suggest much when it comes to the Epcot snacks, we tried snacking around the world for the first time on our last trip, so haven't had a chance to try a lot (sushi in Japan, éclair and mousse in France). But, I'll be sure to include a review after our trip, or pass along any highlights we come across. Only 8 more days!
Good Luck working out your reservations. I don't envy you trying to come up with the perfect meal plan for that many people. We have a difficult time agreeing on where we want to go and there's only the four of us.
