You may have a lot more difficulty changing the airline tickets though. They won't just change a name on it, and everyone must show ID that matches exactly to the plane ticket. There have been people who have had trouble because their ID says William and the plane ticket says Bill. So unless you named your daughter for your sister, that may be a costly problem.
What airline is the ticket through? Some will charge a fee to cancel a ticket (which is what you need to do), and then you can use whatever refund is leftover to purchase a ticket in your daughters name. Some aren't refundable at all, in which case, you lose the money spent.
WDW will be the easier of the two to switch....like Cheshire said, the worst may be paying the extra adult fee.
Admission tickets shouldn't be a problem, because you likely only have vouchers that need to be turned in for a "real" admission ticket anyway. Since your DD is over 10, she'll be considered an adult as far as price goes, so your sister can just take the voucher.
NOW...if your DD has an Annual Pass, or a ticket that she's already used (like a non-expiring from a previous trip)....then you canNOT use those for your sister. Your DD's fingerscan will be on record for that ticket. While some may get away with it, the penalties for being caught can be you're thrown out the park and not allowed to return, and the ticket is taken away (so not even your DD can use it later). Not worth it. So if you have an AP or used ticket of your DD's, then plan to purchase a NEW ticket for your sister.
Have fun! I can't imagine not wanting to go to WDW and get out of school, lol....so kudos to your daughter for thinking of her education first.