What is your MIL is thinking? How could she leave a disabled person in your care right after you've just given birth?! You should be the one receiving care instead.
You are not selfish. I understand your MIL's side too but it is very unfair to expect you to take on such a burden. She's had to take care of her son for the past 30 years and she's probably just emotionally spent from literally half a lifetime of caring for a child with a disability. A 30 year old man who has DS should be placed in a residential facility.
I have a small child who is special needs and cannot imagine placing him in the care of any one else right now. He has not been out of my sight for more than a couple of hours since he was born. No one from my family wants to take care of him, so I can get a break, not even for a couple of days which is all I ask for. I am mentally wiped but I have accepted it and make the best of it. I would never ask a relative or babysitter to watch my child for more than a day. I simply think it is unfair to do so. We are now applying for aid like hospice/respite and other services through our state.
The thing you need to get your DH to do immediately is to help his mother get this young man to a residential facility or another type of assistance. They should look into their state's government agency and go through the process of getting a competent hospice or respite worker to come and care for this young man, not you! I must say I am a bit disappointed in your dh's lack of assistance -- cares so little about you and your newborn's well being? I don't know your BIL, but I don't think you can handle a disabled adult male should he ever fly off the handle...
Please go here and find the services in your mother's state of residence
National Association .... Developmental Disabilities Services
or simply find the Department of Mental Retardation or Developmental Disabilities website for the particular state. If you need some guidance, please PM me...