I'm assuming that you're also from New England, since I think we're the only ones now that get a February school break. My DH is a teacher, so our vacation time is usually limited to school breaks.
We used to visit WDW in the summer, since that's when we had the most time, so we could also visit family and friends who live in Florida. However, in February of 2000, we decided to chance it and fly down during the winter break. We will never go back to summer visits again!
It does get a little more crowded (especially during the beginning of the week when everyone has Presidents' Day off anyway and many people are in town for Daytona), but by the middle of the week it's very manageable. There will be lines, but nothing like those in the summer, and there are no tour groups to speak of. The weather is usually quite pleasant (comparable to our spring), but you might need to dress in layers and bring a backpack with you to carry the clothes that you shed.
The only problem we've ever encountered (and we've gone every February since 1980) was with our flight being delayed coming down once because of snow. We always hope it will be delayed going back, but so far, no such luck!
I'd highly recommend a February trip, but whenever you go, I'm sure you and your nieces will have a great time!
BTW, you might want to post this question again on the Trip Planning thread. I think more people read that.