I really wish I had some constructive advice to give you, but having never gone through this, I'm not sure if what I say will be helpful.
First of all, as most here have said, apologize. If you can get your parents to join you in this, all the better. I understand why you wouldn't want to do this because your inlaws aren't angels from what you've described and could really take some lessons in the art of humility, but here's the thing. It. Will. Never. Happen.
I have met people like this. I have people like this in my life. And I'm pretty sure everybody in this forum (as well as on this planet) knows at least one person who goes by the motto of "Me and mine are always in the right. Everybody else can go to hell."
You can't argue with these people. You can't reason with these people. They are not rational so there's very little to be reasoned with. So please just accept that fact...it will cause you fewer headaches.
Are you depressed yet? I'm sorry. Here's the good news. Though there may not be any benefits that you can see now or even in your lifetime, Allah loves people who grin and bear it. Really. People who exhibit patience, who bite their tongues, who try to throw water on fire instead of adding fuel to it...he loves them. I'm not saying you should become a doormat (more on that below), but try your hardest to not fire back when you're faced with their obvious bad manners. That only works in books and movies. I think Mabrook said it best so I'll just sum up - keep being polite. This will bring you 2 benefits. One, you will have no regrets later in life. You can look back and say to yourself "I tried my best and I did not lower myself to their ill-mannered behavior." That doesn't sound like much now, but trust me...not having regrets on your conscience later in life is a huge deal. The other benefit is this...other people will notice as well. If your inlaws are treating you like this all the time, it's bound to get out eventually. And if they're anything like other desi people (your SIL definitely is because one little comment about how weak you were sent her into an angry tirade), they care very much what "log" will say. Once the gossip starts (if it hasn't already), you might see them quick to change their ways. Or you may not. Some people can't be convinced that what they're doing as wrong.
Now, as for the whole maid issue. This is a tough one. You obviously need some help around the house because it's affecting your health. Your inlaws aren't providing you with one and it seems your husband isn't going to help you out on this score. I'm going to suggest something that's quite manipulative and underhanded. But this is what an acquaintance of mine did. And it worked. And I think it saved her life because she too was like you a few years ago - weak and sick from running after two children and getting absolutely no support from hubby and in laws. Remember when I suggested you should be polite but shouldn't make yourself a doormat? The above advice was the polite bit. This is the part where you stop letting them walk all over you. And it's simple. Do what you need to so that the FIRST set of tasks accomplished every day are for you and your baby alone. Once you are done with those, then do your best to help out around the house. And if anybody says anything to you, well then this is where you're going to have to be brave and have to put up with some nasty comments. This is where you'll have to give them a sugary sweet smile and say, "I know, I know. I hardly have time for anything anymore. But I know you want me to give your grandson/nephew/etc the best care possible right?"
I've never gone through this situation before so my advice should be taken with a grain of salt. But I hope there's something helpful in there for you and if anybody thinks I've got it all wrong, please feel free to jump in and let me know. 'Cause I see this happening all too much around me and I'm at a loss over what to say when people ask me for advice.