I don't know the answer to your question, and I don't know what ships your MIL and SIL cruised on, but I don't think the Wonder's water slide is the longest in the business. The vertical height is only one deck... from Deck 10 to Deck 9. I believe some of the big, new Carnival ships have bigger water slides.
You'll just have to find other ways to claim superiority. That shouldn't be a problem... after all, you're cruising with Disney.