Yayy! Atleast your husband is being understanding. that’s the most important thing. And if he acknowledges that his brothers and SIL arnt very helpful and he cant leave his parents with them, then why cant you, your husband and your inlaws move out and buy a seperate home where you and hubby could have one floor and your parents could have the other? This way it would be less cramped and plus you would be able to chill on you own floor. Meanwhile your BIL and their wives can just stay put?
Why don’t you delicately raise this suggestion the next time you and hubby happen to discuss the topic?
Ps. You sounds like a lovely DIL who genuinely cares about how her parent in laws get treated.