We just got back from our second family trip to WDW. The first involved my entire family (Mother, Father, Sister, BIL, and Nephew in addition to the 5 people in my household). This last one was our family, my BIL and niece. Both of them were good trips with stressful pieces. For my family trip, I fell into my usual role of pleaser and tried to make sure that everybody was happy/fed/enjoying the park/etc. As a result, we did very little because even though I had plans and knew that we should be doing things in x, y, z order I didn't speak up for fear of hurting someone's feelings. My mother and BIL clashed and my BIL often had to step away from the group because my mother is very over bearing and at a point in her life where she CAN do for her grandkids so she does - whether it is wanted or not.
This trip was interesting because we were dealing with the issue of my oldest being sick, dealing with a niece who is not handling her parents (not new) divorce well/struggling with the fact that her mother is an alcoholic, balancing the need for work (my husband) with the fact that the family needed him. BUT, I don't have a defined role in that family group and as such was able to step up and lead everyone through a packed schedule at very busy parks. We still had the occasional melt down/time out (even the grown-ups!) but we were able to step away for a minute and come back together.
I was talking with my mom yesterday about the trip and she told me that she and my dad had a conversation while we were gone. Basically, she was hurt that we didn't involve her/ask her to come along this time and she was whining to my dad about it. He looked at her and said that yes, it stinks that they weren't invited this time but didn't she remember how nice it was to be able to go on family vacations without her parents along? For the first time in forever, she admitted that she can be a handful and then apologized if her behavior is what made us not invite her (it wasn't, but it was still nice to hear.)
Anyway - you're not going to hell for not wanting the in-laws to ruin another vacation. You've gotten some great advice from others and I hope something will make sense for your family. I think somebody suggested renting a house off site for them and visiting there - would that be a possibility at all? They can still get a vacation but not feel like they're ruining yours. They can hang out during the day by themselves or go do touristy things at their own pace and you guys could come over in the afternoon/evening and have a place to chill/visit.
Best of luck!