I think that I have said this before but there is a 5 month waiting period for SSDI, if she qualifies. If she is getting chemo and radiation she could be disabled for quite a while. I couldn't work for over a year and my job is not physical. If she is under 55, they are tougher about approving the claim.
I had no problems establishing a claim with SSDI, based on fatigue which is a well known cancer treatment side effect. I was so weak at one point I was unable to stand in the shower. She should qualiy for welfare and other public assistance programs. The social worker is your ally.

Who's going to take care of her and get her to treatment every day?

The way I got through it was to take out a second on the house which unfortunately your sister can't do.

She needs to take care of this as soon as she can (while she can). Be sure to have copies of all of the scan reports, diagnosis, etc.

Does she belong to a church? Ours has a benevolence ministry that paid a lot of my bills, groceries, etc. Plus I had volunteer drivers and many other forms of support.

She can get substantial reductions in her gas & electric bill, water and telephone. She's probably been eligible for that all along anyway.