B.C.
Your Mom is an adult and I assume of reasonably sound mind.
She makes her choices and if she is miserable it is self inflicted.
You can do nothing for your Mom.
But you can talk to your sister and try to get her to get busy with her own life and not wait on her Mom hand and foot.
It could be your Mom enjoys the attention she gets when she is sick - clearing up her illnesses quickly and without drama would spoil her fun.
If you and your sister take the attention out of the equation, who will your Mom moan to?
Next time you talk to Mom, as she complains about her symptoms, just tell her
"I'm so sorry you feel bad but you know what to do to get better. In the mean time, what are your wishes for funeral plans in the event you let this go too far and it's too late for the doctors to do anything about it?".
After that, make it clear that you and your sister have plans (going to a movie or a concert - something fun) and if your Mom were well maybe she could come too but since she's not, well life/fun will go on without her.
Your Mom needs an interest, a hobby, some friends of her own that will give her a desire to stay healthy enough so she can have fun.