If you are a "Disney Freak" and go a lot, I can't really see why you would be that upset about not being able to tour exactly the way you like. Maybe if it was a once-in-a-lifetime trip... But you probably go a lot. Isn't it okay for this trip to be a little different, a little slower, because you get to share it with your family?
We go to WDW and DL a lot and a few times we have gone with big family groups or with my parents. Those trips are a bit different and come with frustrations ("Get mooooving, people.") But they also supply unique memories that are only possible when you get a group like that together. I like both kinds of trips. It is okay to me that they are different.
This, totally this. OP, I think you need to take off your "MY vacation" hat for this one and put on your "awesome auntie, vacation planner extraordinaire" hat. Your nephew will only have his first trip once, and you get to be part of that! That can be pretty magical...not what you normally anticipate from your version of Disney magic, but magical nonetheless.
I think this can be a fun trip if you manage your own expectations as well as lay out some expectations for everyone else.
We've traveled with family and also with friends. Recently we've traveled with friends who hadn't been since childhood, that was a lot of fun! To see things anew through their eyes was a treat, but I certainly had to change my mindset from what I normally expect when it is just my immediate family or just DH and I as a couple. If you can go in with completely different expectations it can be a ball! But if you go into it expecting everyone to do what you always do, it can be a big drag.
Something that helps a lot for me is to lay out a planned schedule, have it written in a spreadsheet or powerpoint, and share with everyone before and during the trip. Kind of like "this is what I have planned, you can join or you can do your own thing, and feel free to meet up later if you feel like it".
This also might be a fun opportunity for you to "borrow" your nephew and try out parenting for an hour or two while you let his parents have some alone time. At that age he may enjoy looking at the ducks or finding a flower or something...the little things that you might not even notice on your regular trips. They focus on the funniest things at that age, and that's awesome! If you manage your expectations and keep an open mind, it could be a super great experience for all of you!