Sorry you had such a bad experience. Don't take it personally, but rather as a learning experience to never count on the mouse for communications. You really can't compare your sister's issue with your own, those are two entirely different systems. Makes no difference if she's staying at a value or deluxe, good to hear Disney did right by her.
Your issue is all about DVC's communication of the event registration. Again, it has nothing to do with MDE.
Draw up a letter to DVC, address it to whomever is in charge these days (I lost track after Claire). Tell them your experience and how you feel they can fix the issues you experienced (not how they owe you as that is a quick way to the circular filing bin). You may be surprised by their response, fingers crossed for you. My key points would be communication/miscommunication. For a company who thinks of themselves as highly technical, why is it they can't get simple mailing lists and dates of visits right? Those are the building blocks of business right there.