Chris,
I had similar problems to your sister trying to get travel insurance to America for this years visit to Sebring, due to extra ailments that have come up over the past year.
I found that trying to apply online does not work if you have serious health problems.
The only way is to contact the insurance companies direct, although I had many rejections, but in the end found someone to insure me at a sensible price.
Also if your sister is trying to insure as a single trip the cost will be considerably higher and likely to be rejected.
The best way to go is to insure annually. For some reason the policies are cheaper and easier to obtain.
You could try the company i am currently with
www.secure-travelinsurance.co.ukAndy