Honestly, with a 4 and 6 year old, I'd say no to the BC. Too much hassle. I honestly think that it's geared for the older kids. Yes, I think that the kids are ultimately going to be who you have to plan around. The pool is huge and your sister would end up having to corral the kids while you and mom want to have a drink, sunbathe and float around. The six year old will want to try the slide which isn't even in the pool area... then when you want to eat, the BC giftshop is the fastest choice but there's not much in it. It's not as easy as you think having to drag two tired children into Epcot... for Mexican... so you end up haulin to UK for fish and chips every night to bring back to the room cold.
I guess if you go into the trip knowing that you'd be eating (and paying for) room service, scheduling around dining reservations, or relying on the good faith it doesn't kill everyone to head back into Epcot everynight for dinner, sitting with toddlers at the kiddie pool, and picking up everyone from the BC, The Boardwalk, Swan and Dolphin before going to the MK... then y'all are good.
I'd definitely choose somewhere else this go.