Sean Turner, 25, was found dead Sunday night at the San Diego area home he shared with his grandmother in National City.
Turner, who is the stepson of defendant Keith Harold Turner, suffered from acute diabetes and had been in the hospital almost once a week for the past year.
Turner testified for the prosecution in the trial of his stepfather, convicted last month of voluntary manslaughter in the death of his wife, Toby Turner, 44. Jurors discredited Sean Turner's testimony, noting his variation of stories and a prior drug history.
The elder Turner is to be sentenced later this month.