Advice on applying for disability...l/m

I will try to be brief here.  It is hard to put details of an extremely complicated life situation into words.  My sister and I are in our 50s.  In September, my neice committed suicide.  Oh my, even typing that makes me cry.  She was one week short of 27 years old.  Not sure why this happened but that's another discussion.  My sister was absolutely shattered.  Got a call today that my sister collapsed at her job and is now in the psych ward, under sedation. 

I live about 12 hours from my sister, two states away.  I talked to my other niece, aged 21, who asked if she and her mother could move over here.  My niece is working full time and in college to be a surgical tech.  She stopped school because her mother is basically needing so much.  My niece wants to move here so that I can help them out.  What a blessing this young lady is.  She wants to return to college and support her mother all at the same time.

My sister's health is not all that great - she has bilateral knee replacements and von Willebrand's disease.  I think this will also be long term mental health treatment.  I would lke to get Sis on disability with SSI.  Her employer has basically tried to be kind, but they have basically told my sister that she is not doing her job.  And now after this collapse, I can imagine that my sister will not be employed long.  My sister used to be an RN but can physically no longer do the job, so she was working the front office at a doctor's office.

I need advice on how to apply for disability or if this is even doable.  My neice wants to rent an apartment near me, so she and her mom can live near me.  It would be nice for my sister to have an income without worrying about working for a while.  She has no assets, little savings, no house, no car.  Basically, she spent every dime she has made.

Any advice is greatly appreciated.