I guess the thing I'd ask is how important it is to have this trip go "your" way and how much you are willing to compromise for the sake of the trip. For us, the time for trips is usually a bigger issue than the money, so no trip is "free". It's totally reasonable to say "I can only take one trip, so when I take that time away, I want to make sure it's the one I really want." If going on the trip is not going to otherwise affect you, though (e.g. taking the place of your next planned trip or something), I'd encourage you to try compromising. Go in realizing that it's not "your" trip, but it's one you are getting to come along for, and will probably get to have some fun stuff out of. If the only way you will enjoy the trip is to do it your way, and others will have to adapt to you, that won't be good either!
It seems that each of you has a few things needed for this trip to be acceptable:
1) Mom: 5 TS meals together, plus everyone together for Mickey
2) Brother and SIL: Nap times for your nephew.
3) You and husband: Trying new restaurants.
4) Other brother(s): ???
There are probably more, for each of you, and you should make sure that you really have a good sense of what will and won't be an expectation for everyone. For instance, if some of them will be the "I have to sleep in and not make rope drop" types, you need to know that at the outset. If some are expecting to all be together every minute, you need to know that or have them change that expectation. Or, if your Brother and SIL (or others) have a requirement of "must have character meals each time" or "everyone has to leave when we do for our kid's nap", then that really might not work with your priorities. They probably have a "need to be back by X p.m. at night" requirement also, but might not have said it yet... But likewise, you need to think about whether you not doing the parks exactly the way you want to is going to "ruin" the trip for you. Will you be OK moving at a different pace, maybe going to a few attractions that aren't priorities or missing some outright, scheduling restaurants that the nephew (and others in your family) will be OK with (thankfully, most Disney restaurants are at least OK for kids), scheduling so that you at least meet up at the right times, not doing every activity that you would like?
When we've done parks with others (not WDW), we've never done things the way we'd do them on our own. But, we still had fun, and getting to spend time with friends or family made up for all the "missed" opportunities. Basically, the key is to figure out what each of you _really_ needs for the trip to be good, and then see if those are compatible. For instance, I am a roller coaster fan, and I've had to sometimes say - I want to ride roller coaster X (and then found a way to do that in the visit), but then deal with not getting around to riding roller coasters Y and Z. If I could not have enjoyed the park without doing X and Y and Z, I wouldn't have enjoyed the trip.
As someone who kept our kids on strict nap schedules (even on a Disney trip once!), I sympathize with your brother and SIL's concerns, so I am a little biased there. If they tell you that's a priority for them, make sure you understand their schedule needs (the good thing is that it really will be a schedule, that you can plan around). If they've been keeping a strict nap schedule, they are already very very familiar with the "I can't do X because of naptime" issue, so they will probably understand very well that they have to give up some things for the sake of the naps. Don't try to tell them "oh, I know lots of people just have their kid nap in a stroller or during a show, yours can, too" - for our 2 kids, one of them NEVER napped in a stroller, period, and the other one sometimes could, but never well/enough. There are good points and bad points to strict nap schedules, but one of the bad ones is that if you don't follow the routine, it can really throw things off...