The 43-year-old is known to have broken all her ties with the family since 1997 after she appeared as a porn star in "Boogie Nights", reported Sun Online.
"Joan and I have endured many sleepless nights questioning how and why it all went wrong. To this day, we honestly don't know. As a family we were incredibly close, Heather would always share everything with us.
"She really was the perfect daughter good at school, joyful, loving... she never ever gave us a single problem.
But suddenly, almost overnight, everything changed and it is a mystery to all the family how a girl who loved us all seemed to distance herself for no real reason," Jim said.
"Not a day goes by when we don't miss Heather. I just hope that one day the phone will ring and it will be her voice at the end of the line," Joan added.