I would stress to your sister in law how much less walking you have to do at DL compared to WDW. Rides are much closer together and you can parkhop to DCA by walking across the Esplanade, unlike WDW where parkhopping requires riding busses, boats or driving. Parkhopping is almost a must at DL because you can really maximize fastpass usage this way. Also there are many close hotel options (both onsite and offsite) that are a 5min walk to the entrance.
And while you are walking less you are also much more comfortable because its not insanely humid like Florida which means no summer thunderstorms either.
While some rides/attractions are better (space, Fantasmic etc.) , some rides are worse (Pooh, Jungle Cruise, TSI etc), many are close in quality. Overall, however, I think the DL rides are better.
I think DL is much much cleaner than WDW. And I may get flamed for this but I think the CMs are nicer at DL as well.