I think this is a very smart plan and you would be wise following it. I have gone every year to WDW for the last 9 years and this plan is the way to approach having 5 days.It depends... honestly, since your sisters are so young, you could probably do a day and a half (maybe even just one day) at Epcot and not have to be so rushed at HS or AK. If they were old enough to drink around the world or young enough to really get into a lot of the kiddie activities over there, I'd say you'd want more time at WS, but otherwise really a half a day in FW and a half day at WS might be enough. AK has some neat shows, and they might want to see some of the shows at HS as well, so even though those parks are lacking on rides compared with MK, they may want more time there.
What I might do if it was me is plan this way:
Day 1: MK
Day 2: Epcot
Day 3: AK and HS (maybe go for the evening to experience Fantasmic and get in a few other things)
Day 4: HS (do what you didn't get to the night before) and then back to MK or Epcot (based on what everyone want to do)
Day 5: Whatever you all decide you want more time to do!
Build in a little flexibility so that you three can go with the flow.
I hope that helps!
ETA: the reason I suggested doing a full day at MK and a full day at Epcot first is that this would give you a better idea of how you might want to spend that last day and a half or so... that way you're not completely waiting until the night before to try and change fastpasses, etc.
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