Okay Ive read OP's updates and... I've come to the conclusion that this is not acceptable behavior.
You say these two in-law women have digestive issues (of some description),
That they know there are certain foods that might set them off but they do not avoid the foods that give them the rumbles.
Being quite polite about it you've said that they're food pigs, that they eat and eat a lot.
Then again, being polite about it, you said that their favorite activity post meal is to retire to the sitting room, lift their cheeks, and try to outdo each other with massive toots... So this is actually more of a form of entertainment for them than embarrassment
You say your husband is great, and you don't have a problem with him, but he sees nothing wrong with his relatives behavior, and in terms of hosting a meal, he can't do anything to help cuz all he knows how to do is load the dishwasher (but has many other admirable qualities)
You've said that when you guys go out for a restaurant meal, they don't do this behavior. So in other words, it is controllable.
Reading all of this, I would be running out of sympathy for this pair, also.
Breaking wind is a natural digestive process, we all do it, but I'm sure you're trying to raise your children to be polite in social situations and that would include, if you've got to do it, try to go somewhere private and if you must in public, to excuse yourself. That these two wind bags are setting a bad example for the kids.
If they genuinely had some kind of serious medical issue, and really were trying to avoid their trigger foods, and were trying to be polite about it but couldn't help the flatulence, then I would say you're being prudish.
The way you describe it, they eat everything in sight, don't care for the consequences and, although they can control themselves in public, they think it's funny to faff in private. I've never been a connoisseur of fart jokes or fart entertainment or fart amusement. That's really more of a prepubescent boy thing.
Yeah it's a real shame that you've got this pair is in-laws.
Just asking, aside from this issue, do you get along with them pretty good in other situations? Like are they supportive of you and your family, accepting, interested in your lives, involved, respectful, all those good things? Or do they just like to come to your house for a big meal and a farting contest?
I'm sorry you have to put up with this. After reading all of your updates, I'd have to agree with you. It's unpleasant and unnecessary and I don't blame you for not wanting to have them as guests