What can I do to make my mother get medical attention, suffering?

I live with my other half and his mother in her house we moved there because she needed someone there with her. Help her etc. The problem is she will not do anything to help herself medically. She is sick and has had recurring sepsis for over a year. It has and I'd damaging her kidney. She always tell them she wants to leave once she starts to feel better. Everytime it is good for a very short time then I can see the signs slowly coming back. Her son and his sister will not take action. She is obviously not capable of making a sound medical decision if she over and over does the same thing knowing the outcome . The sepsis never goes away. I am scared. How can I get her son to see that if he doesn't take control then she is going to die, very painfully. What can he do to ensure his mother stays until all is gone.