I live in PA too, but in 7 days we are taking a plane instead. Where in PA to you live? I live in Danville. We have drove to Disney many times though. Are your sisters younger or older or are you a middle child.
If they are older: they may bring some type of interactive phone (smartphone, iphone, ect.) There is also the option of bringing some fun games like mad libs, or even planning your trip if you haven't done that already.
If they are younger: You can be creative and make games for them such as bingo (which is what I did by making disney homemade bingo cards) or crosswords.
If you are the middle child: You can do any of the previously stated ideas above or you can take turns watching movies or choosing car games with your siblings. I have drove 18 hrs before with a 13 yr old sister (expeditioneverestgrl) and a 11 yr old brother and used these ideas before.
Hope these ideas help!!