How is your MIL for communication? How many days did you go with her? Because she doesn't seem to want to do the same sort of vacation with your DB... Maybe start with talking to her about what
she wants and let her plan the vacation, then once you see what is important to her, come back with suggestions to make everyone happy. It's hard to give advice when there are so many details we just don't know about your family. Family dynamics are so different.
Assuming she wants more relaxation, doesn't like the parks so much, and likes pool time, maybe you should do a classic touring plan strategy. Have an early breakfast together, maybe even sometimes an 8am ADR in the park for a 9am opening (depending on when you are going). Tour the parks in the morning, and then leave around 12pm. Grab lunch at one of the nearby resorts or your resort, and then hang out by the pool/take naps in the afternoon. Get changed for dinner, eat together, and then go to a park for some more rides and the night show. Go to bed on the early side, by around 10pm or so depending on the show schedule, and the do it again. It can be win-win. So long as your family is not a bunch of night owls, then getting up early and getting to the parks for rope drop will give everyone the chance to see a ton of attractions with shorter waits (who doesn't want that), you don't have to stand in a mile long line for food for lunch, because you will be out of the parks, and often get a better lunch. MIL has a LOOONG break in the middle of the day relaxing, and the kids have fun and get refreshed. You have a nice dinner together, and by then everyone has the energy to hit the parks again. Who doesn't like fireworks?
If you are a bunch of night owls, take the opposite approach. Sleep in, eat a leisurely breakfast, hit the pool or go shopping. Have a late lunch and then be in the park for the afternoon parade. Have people save seats while others grab Fastpasses, and then enjoy the parade. Se some of the "anytime" attractions with shorter lines, grab dinner, and then party until the park closes, again taking advantage of the short lines at closing.
MIL gets pool time, everyone gets park time with shorter waits and cooler weather, and you get a lot of together time.
I did
Disneyland with my DH's family. He comes from a family with 6 kids plus spouses and grandkids, just to give you an idea of the craziness of just managing a vacation like that. Yes, because we did not have private sleeping arrangements, I did at one point discuss not coming on another vacation with his family until we could afford to pay for our own room with DH during that trip (we didn't have the funds at the time to pay for any vacations as DH was still in grad school). However, his family is very important to him, and saying we couldn't go on a trip with them just because of some uncomfortable circumstances wouldn't be fair to DH. So now I just view family reunions as fun reunions- I don't pretend that they are for me to enjoy things I want to do. Instead, they are for me to enjoy time with my husband's family.