We typically go 3 times a year and we live in Ohio!... Big difference is what we pay in airfare, compared to you. On average its only $250 round trip and 2 hour non-stop flight, so that makes it easy to go.
I plan on moving down there in a few years and do not think it will spoil the magic because I will not work for the mouse... I think that makes a big difference if you get burnt out on the World. You start to see too much of the behind-the-scenes stuff.
Your sister only had 2 parks for the kids to grow up in, (possibly only 1 if they were young and DCA wasn't open yet) where the world has 4. I think that also plays a factor in the kids not wanting to go to DL.