k im no expert here, but all i can say is that as long as you are living in your in-laws house, then this behaviour is all in likely to continue. They're word is law and whats happening here is an issue of authority, where your MIL expects you/anyone to conform to her (perhaps unreasonable) expectations, abide by her rules and play it how they want.
Now thats all good and well seeing as its THEIR house and they've every right to lay down law as owners of the house, however its pure unlucky that they're behaviour is so appalling and uncompromising, that it seems to have got to the point of breaking you down.
I think something you should consider, is if this criticism is directed just at you or in general extended to everyone. i say this because you say your husband is used to it from a young age, your sibling- in- laws seem to have accepted it as normal behaviour to extent that they are exhibiting similiar tendecies, hence maybe its deemed normal behaviour with in your in laws, and as now member of family, it has been extended to you?
If this is the case, rightly or wrongly, it helps to know if its been directed at you not for you being you, but because its just a norm in the family. It helps to know as the latter makes it slightly less personal/painful and a tad easier to tolerate and make a stand.
secondly, it might help to ask your sister in law about her experience living in the house when she first came, because no matter how much they praise her in comparison, it is likely that she went through the same thing, and most probabyl the main factor why they moved out and so quick to hand over responsibility to you guys.
I may be assuming things, but you never know, you might even find a person aside from your husband, who you can confide in and most probably the only person who knows where you're coming from. So dont see her as the competition your in laws are making it out to be, but do one better and get her on your side and your MIL comparison tactics should fall flat on its face when you both know the real facts on the ground.
But as i said, at the end of the day, no matter how much you do for them, it will be seen as thier God given right, and not as your good will, because after all you are living at thier place. Any complaints/talks will be seen as an affront and ungratefulness on your behalf, and will probably worsen the situation.
I think your best bet is to make a stand and get out. You will not be treated as a seprate individual/couple until you do. You say they rely on you, what is it they rely on you for...i am assuming its more financial support than physical care... ?
it is in your and your hubbies best interest to move away a'fore you're broken apart, and no matter how wonderful your relationship with your husband is now, bottling it all up, will eventually take its toll. you need to confide in him directly, and he should be man enough to take it in, and make a stand for your sake, for his mothers sake.
i have much more to say on this, but i think ive said enough considering, i dont know the full details. But whatever you do, just keep in mind, that shari wise, it is not your responsibility to take care of his parents, it is your good will that you are doing so. But dont let it go for granted, not at the cost of your nervous breakdown, nor at the cost of your marraige. So if not for your own sake, for your marriage sake, take up your husbands offer and move out.