You will have a great time with a 6 year old. As the mom of a 6 year old, I just want to add a couple of things.
First, although the 6 year old might be tall enough for most rides and attractions, that does not mean that they are either appropriate for her age (e.g. Alien Encounter) or her desire. I would never force a child to go on a ride if s/he was not comfortable about it. For example, my DD does not want to experience Tower of Terror, nor does she want to do Rock n Roller Coaster. She is tall enough for both, but ToT would freak her out and RnR's restraint bar knocks short people's heads about -- it hurts! On those kinds of rides, ask to use the "baby swap" (only call it a "kid swap" in front of her). She'll just have to sit out the rides when you and your sister go on them separately.
Second, there is a lot of walking that goes on at the parks. Epcot, especially, is huge. It is very tough to walk it all day and night on full grown legs let alone little 6 year old ones. She might want to start out without a stroller, but when it gets tough and tiring you should get a stroller and use it. It's also helpful for loading backpacks, etc. when you're tired of carrying them.
You're going to have a great time.
