Sorry this is long but please take time and read it, I do have some good advice and maybe help you to convey to the others in the household.
They will retaliate on MIL so this is a fine line that you have to consider.
I don't know what I'd say except I don't mind burning bridges with people like this and I'd flat out tell them off right there in front of MIL, FIL, and God himself.
They live like this because your MIL allows it. She takes care of them and they accept it. It's her problem and you see how it's taking its toll on her. I understand how awful they are.
I took a class at OSU when I was in college called Death and Dying. Chuck Edgley taught the class. It was a Sociology class that showed a lot of the stress and mourning that happens when someone dies.
There were basically 4 areas.
1. Expected death at an expected time
2. Unexpected death and an expected time
3. Unexpected death at an unexpected time
4. Expected death at an unexpected time
1. Someone has a terminal illness and is given 4 weeks to live, they die at 4 weeks.
2. Parents are supposed to grow elderly then pass away after a long long happy life.
3. A child passes away from an accident. A sibling dies in an accident. A parent dies from a sudden heart attack or stroke. Totally unexpected, a phone rings and they're gone.
4. The hardest one of all. A person has a terminal illness and is given 4 weeks to live. They deteriorate and wither away but just don't die. They are sicker and sicker and sicker but their body hangs on. They are still there needing total complete care 6 months later. You are exhausted, beyond all stress with everyone coming and saying their goodbyes and then coming back a few months later to visit again then the person is still there even after 9 months, withering away and sicker and sicker.
The caregiver has guilt over their contrasting emotions. I'll miss them horribly when they're gone and they're still here and I'm so happy I still have them. Then when it's the middle of the night and they're having to get up and clean a bed full of poop and puke and having to lift that person out of it all and putting them in a wheelchair covered in towels and strapping them in. Then taking them to the shower, washing them, scraping poop and puke off their bodies then off the chair and stuff. After that they have to change the sheets and put them back in bed hoping they'll not do that again tonight so you can just get some rest for tomorrow. Then you have to go start a wash load and run it at least 2-3 times to get all the debris out of the items. Cleaning the washer out between each load.
It's time consuming and draining of a person's spirit. The conflict of wishing they'd just die and be free of their pain and their dying body and you just want to get some rest for a change....and it goes on.
When they finally do die you have guilt over those wishes they'd just go ahead and die. You have the deep dark secret that you wished they just go ahead and die and you can't tell anyone because that's a horrible feeling to have but you did have it. So they live with that guilt. AND they miss their family member, who they were before they got sick.
So she needs support and love and compassion. Is Hospice helping? They'd get those in that household moving I think.