@kat4321 Since your daughter says she does not want help there isn’t much anyone else can do. It’s a very difficult and sad situation. Does your son-in-law see a mental health therapist/psychologist? If not, then this would be a good idea as he needs not only his own support but also a therapist can help him to figure out how he wishes to handle his family situation. Presumably, your son-in-law is major caregiver in the household when his wife is ill and in bed. This is a huge strain for him.

Would you like to ask your son-in-law if he is seeing a therapist or would like to see a therapist?

In my healthcare system the hospital has classes you can take for CBT or DBT. I think our public clinics have them too. I know family services does.

These are generally once a week small group educational classes. It might be a good entry point. Some programs you can drop in, some have a commitment of 6, 8, 10, 12 weeks depending on the cluster of folks.
When I went to a program with my daughter she didn't talk the entire time, but we went every week (it was a family program centering on adolescents. She could go to an adult program on her own, or someone could take her and sit in the waiting room to start).
My daughter continued seeking help after her participation and she has definitely improved her ability to self regulate.

There are caveats: our entire family participated in therapy. It didn't make sense for her to come home and reenter a toxic atmosphere that was working against any progress she made (mostly me); she went through several diagnoses and it took years and patience I didn't know existed; and I had to completely abandon my expectations and learn to provide what she actually needed.

And by any measure my community called me an exceptional mother before and during the onset of her illness.

@kat4321 I am and have been since 2012 in a similar situation. I have had to raise my two grandchildren since, although they are now on their own and over 21 years old.
I get what you say and what you are going through. There is no question it is a difficult situation for so many reasons. There isn’t a single and easy answer and it is near impossible to “make” a severely depressed person do anything that a non depressed person would do. Sometimes there aren’t clear cut permanent solutions. As much as some of their behaviors are often selfish in nature, they are many times the product of the illness and not a lack of care and love for the children. I am sure she would give an arm for them if she had to. Having said so, of course it is paramount to protect the children. I hope you know you are doing what any loving and responsible mom would do for her adult child. I hope you take care of yourself in the process and I hope you know there are lots of people in the same situation. If you are able, remember you are doing your best. Peace and love to you.