My SSDI was $1500+/mo (now I have to tell you that I have had some pretty decent jobs over the years and was fully vested in the program so they awarded me the maximum amount) and I had no choice - Social Security decided what program I was eligible for. I simply made an appointment, brought ALL of the required documentation (and then some), medical records (My doctor agreed in the beginning to give me a copy of every test, lab report, scan analysis report, etc.)and a decision was made in 2 weeks.
After 2 years you are automaticaly eligible for Medicare. The key here is to dot every "i" and cross every "t". I even brought in my staging information, NCI survival odds statistics, etc.

Being in regulatory affairs for almost 30 years I have learned how to deal with the government. It is far better to go in there very low key but with impeccable documentation. You might shoot yourself in the foot by being a squeaky wheel. If they deny your claim, the path is long and arduous so be very careful. You can go on line and get prepared ahead of time. http://www.ssa.gov/disability/

The basis for my claim centered around cancer fatigue, a well known long, term side effect, from radiation and chemo. I was too weak to work and had to have a driver and caregiver (and I was VERY healthy going into this). I also told them, ahead of time, that I had a compromised immune system and could not allow myself to be exposed to the other people in the waiting room so they would have to send someone out to get me. They sent a guard out to get me while I waited in the car. Your eligibility date starts, usually, with the date of diagnosis and is retroactive to that date so you already have some of your waiting period over with.

There is also a program called "in-home supportive services" (IHSS) where the caregiver gets an hourly wage for a predetrmined amount of hours each week. This even covers things like shopping, laundry, housekeeping, errands, meal preparation, etc. - Even spouses are eligible. This is usually administered by the county.

One of you has to have a calm head and it probably isn't going to be your sister (the patient). So breath deep, take it slow, be thorough and complete.