If someone is cremated with no autopsy and you suspect foul play how do you prove it?

An elderly woman I know said while alive that she never wanted to be cremated. She was going to live with her daughter in Ohio, and when she died she wanted her body flown in her coffin back to San Diego to be buried next to her first husband. Her two wills stated that request. The first will makes both her offspring executers to her estate but after living with her daughter in Ohio for 2 years she changes her will to make only her son the executer and the daughter is now the beneficiary only. When this elderly woman dies, several years later & I obtain a copy of her death certificate it says she was cremated without an autopsy & her request was not honored to return to San Diego in her will. This elderly woman came into alot of $$ living in the same town as her daughter . The funeral home would not respond to a grandson in another state when they were asked if this elderly woman was alone when she died. So, with the body cremated how does one prove foul play if this was murder?