@monkey2009: I am very sorry about your sister's passing.
To try to answer a couple of your questions. No, dialysis is generally not painful. It takes three hours or so to do, but usually does not cause pain.
Renal failure causes toxins to build up in the blood, and this can certainly affect someone's mind. Your sister probably was convinced she saw the snake, but it was almost certainly due to the toxins in her blood. They affect the brain.
The vomiting was probably caused by the toxins in her blood, as well. This can cause extreme nausea and all sorts of other symptoms. Because her kidneys were unable to eliminate the toxins in her blood, they built up over time and probably caused most of the symptoms you witnessed.
Please try not to feel responsible for what happened to your sister. Doctors usually cannot administer medical treatment against a patient's wishes, no matter how much her family would like them to. And you can't convince someone to do anything if they are determined they will not do it.
Please accept my sympathy and condolences in your loss.