dabee,
i think you should not haf negative feelings about your MIL.... she's not the one to be blame for all your unhappiness... i feel like it's more on those selfish siblings-in-laws of your...
your MIL is jus like you said, a mother hen who is always on the lookout for her babies... ensuring that they have sufficient to eat, sufficient to spend....
my sis's family (sis, BIL, niece) goes to my mum's place for dinner everyday as well, and my brother and SIL almost everyday.... but we all got along well with one another and we take on responsibilites to give allowance to my mum....
that's why i say it's more like your siblings-in-laws are the ones trying to be funny and create unhappiness....
as for your MIL's living habits, maybe can slowly talk to her and see if haf improvements...
oh, maybe can also consider not to stock up too much stuff at home (like tissue papers, toilet papers, ...) so that those irritating fellows cannot get with their way....