PSquared is right.
Right, being the alternative perspective (that I love being!) I think everyone has missed the main culprit/dude…Laila143’s husband. He is going to be feeling an immense pressure and responsibility towards his parents. First off, thats A GOOD thing, because it shows he has a heart, is loyal and grateful for the upbringing he recieved.
Secondly, look at the positive instead of as an impending doom. You can REALLY use this to your advantage Laila143. You had an opportunity to understand life living with them, what you didnt like and now you can effect change for the better…now that their living with YOU.
But before going into this, for EVERYONE ELSE, please understand and stop having a go at her for feeling like this, because let’s face it, she’s human and clearly balancing motherhood, housewife duty AND a decent career as a Doctor, so hats off to you Laila. If anything, you’ve earned a right to have a say here. You can’t blame her for having fears, she used to live in hygeine nightmare with zero ambition elders who didnt raise their grand kid appropriately when entrusted with that responsibility. (agreed its HER son, not theres, but we’re Pakistani for crying out loud, we’re responsible for the hungry relatives ‘back home’ but not in raising your own grandkid?! It’s just a meal and a quick change of clothes?! MIL’s shouldn’t take the responsibility if they aren’t up to it, period. Despite this, Laila’s still grateful for even that much? So end of that please.)
Ok, speaking to hubby is not going to be easy, as he’s already stated his position. Thats very interesting, because I get the feeling he knows you won’t be happy with this. But you dont have to be a martyr in this…ie;
Tell him you will help him in this decision and will do your best for his parents as a DIL, but you need certain things to ensure this is a successful ‘conflict free’ venture;
a) HE has to help raise your son so rotas for bathing him, clothing duties and feeding MUST be shared.
b) MIL and DIL are a JOINT responsibility, therefore whilst you are both working to provide for the house, you’re duties towards PIL’s do NOT replace HIS responsibilities towards them, hence their upkeep, trips, food etc MUST also be shared by him also. PLUS he needs to allow your parents to come over more often too (to stay) as you said you would like that and perhaps it can give him an opportunity to experience what you are going through…
c) Let him know this will not be easy, re: reduction in space and privacy, therefore you want to retain autonomy in areas that are convenient for you (to get by) ie clothes, social time AND considering he has FORCED this decision on you (‘‘no ifs and buts’’?) then cos you are accepting this, he MUST equally allow you the final say in how the house is run. This is important as 2 leading women in one house is pure tsunami/earthquake/jahanum/ in one.
It’s essentially both your house, so he should have been more understanding or atleast stated the case with more consideration towards you. But when pressurised with guilt (that I reckon he has) then it’s understandable why he’d do what he’s doing. Work with him, understand him, and you’ll see he will increase his respect for you…and no one says you dont have to be shrewd about it 
Remember, be astute, and if needs be, sit MIL down and tell her how the house runs, how you’d like her to help you run it ie the child’s religious and educative aspirations, welfare are paramount. Thats one common ground between all of you.
Sorry for the boring email, but hope it helps!