i would say, DON'T say sorry to them, it will just make them feel that whatever they did was right and whatever was done from your parents side was wrong (which was wrong btw)... but as we all know that you are dealing with such an immature crowd of ppl, apologizing them will only boost their ego and they will come after you making you say sorry each time you guys will have an argument, no matter if it would be your mistake or not. Remember, being apologetic when it's your fault is a very brave thing to do, but saying sorry when it wasn't actually YOUR mistake is just not right.... it's kinda like sending them the message that being offensive to someone is a right thing to do.
the better way is to IGNORE their childish attitude and be cool with them, talking and acting normally like if you are naive and aren't really paying any attention to their bad behavior. If your husband keep insisting you to say sorry to his family ask him to apologize to your parents on his parents behalf as well. Both sides were at fault and both of them shouldn't have interfered with your family (comprised of you, hubby, and the kiddo only) matters. If your hubby didn't want to hire a maid it should be the decision of your family as a whole and not like "mera ya tera" decision... and your parents should have been fine with it. Although, I don't think your hubby will agree to say sorry, or maybe he will if he has gotten some brain, but he might atleast stop pushing you for a sorry as well, i think.
the whole issue started with a maid, i am also wondering why exactly did you need a maid anyway???? you have just one kid and you already are trying to avoid your role as a mom??? your kid is your responsibility and if you or your parent think you are just too little and not ready to take charge of your baby then you must have not gotten married or planned a baby in the first place. But if you decided to be a mother anyway then it also should mean to ACT like one. Me and many mothers i have seen are raising their kids without a maid or any extra helping hands... we do feel overwhelmed at times but none of us have died yet by the burden of motherhood. Just keep one thing in mind, all of this exhaustion and depression would be over in a couple of months when you and your baby will get used to each other and your hormones will bounce back to the normal level.... some sleepless nights after having a baby is not a life long thing.
PS: maybe your karachi wali SIL is right in taht your baby isn't getting just enough food to sleep through for a couple of hours at a time. How old is your baby???, if he is a baby baby and hasn't started his solids yet and is exclusively breastfed, try adding a couple ounces of formula milk. this really help fill their little tummies up. Or if he has started his solids already, try giving him baby cereal three times a day. Or it might be gas or something. Best thing would be to consult his pediatrician.
Good Luck with everything and enjoy the babyness of your baby!:)