Elvis's mother Gladys died of a heart attack at age 46. However, a complicating factor was liver failure, which was attributed to excessive drinking. In addition to her heavy drinking, she suffered from the effects of taking diet pills.
Elvis' father Vernon died of a heart attack at age 63, two years after Elvis's death. It was reported that he never got over his son's death and "died of a broken heart".
Elvis's twin brother Jesse died when they were born. I have read some reports that he was stillborn and others that say he survived for a few hours. (Elvis was the younger twin, by a half hour.)