Mrs - can my American sister stay with us in the UK?

Hello, I would like to ask your advce in any way. My sister is from Arizona usa. She is 50 years old and was diagnosed with stage 3 breast cancer in 2011 and spread out in other organs like lungs, brain and throat.. She still having a chemotherapy and just to prolonging her life. She's very positive and looking forward to have a normal life. She have no family to be with during her battle,only a kind friend with family to look after too. 

My question is:

Is there any chance or what are the possibilities that she can stay with us here in England with all her medical insurance??