A few more 2 cents from an old man...
In regards to the SIL, all I'll say is I've been considered the a-**** in my family because I won't go to Disney with other family. I've tried to explain in the most polite way I can that a Disney trip is too expensive to feel like you're compromising. For that reason I've always preferred to do our own thing.
That said, I have found that different trips can be enjoyable for different reasons. Our first trip with our son, when he was 18 months old was great. He loved the music and really had no fear when it came to any of the rides he could ride and characters. Our second trip with our kids, when our daughter was 2 1/2 and our son was 3 1/2 was probably my least favorite trip. We had an okay time that trip, but the kids really needed more rest, more need to find bathrooms (our daughter got a new outfit because we didn't make a restroom in time) and the kids were real hot and cold about characters (some we thought would be favorites scared the crap out of them). We didn't make another Disney trip until the kids were 10 and 11 and that was pretty much the perfect age to me. Both were into the characters, could ride any ride they wanted and really enjoyed every aspect of that trip. Since then we've been on band trips when it was more about seeing them march and perform, rather than how much we got done.
It's true a 2 year old likely won't remember the trip, but that in itself isn't a reason to leave them behind (to me). To me the key is knowing what your kids can tolerate and make sure you stay within everyone's limits. The saddest thing for me to see at the parks is a family meltdown when you know the kids (and the parents) likely have just been pushed too far without rest. Letting the kids (and old folks) nap when needed will make everyone happy in the end. You certainly want to get your money's worth, but not at the expense of really enjoying yourself.
Whatever you do, do it because it makes you and your immediate family comfortable. At that point what other people think really doesn't matter.
Thank you!!! YES, they key thing is to realize that there are different approaches for different people. It is a FAR different trip with a 2 year old than it is with OLDER kids. As long as you know this at the start ....and as mentioned above, know everyone's limits and tolerance, you can make it enjoyable for everyone! It is NOT easy to be at DL with younger kids as pointed out above...lots of demands!!
....and too many assumptions have been made here about the SIL.....(selfish because she is working out plans for HER family??)........ it is very difficult to figure out how to please everyone in ONE family, let alone TWO or more families!! Meet up periodically, go on some rides with the whole gang and then split up again when the older kids want to do more. Oh, and yes, do take full advantage of the rider switch!
AND.....most importantly......HAVE A FUN TIME!!