Sharon's Tate Sister Speaks about Manson Murders

08/29/2014 AT 06:35 PM EDT

It was an unusually warm day in Southern California on Aug. 9, 1969.

The Tate family had just moved from Northern California to the Los Angeles area, where their oldest daughter, Sharon Marie, 26, was winning fame as Hollywood's latest "It Girl."

Married to acclaimed director Roman Polanski, Sharon was 8½ months pregnant with the couple's first child, a boy, and the family was eagerly awaiting the birth of the baby.

"Sharon was my mother's world," Debra Tate, then 16, remembers.

"And she was my sun and my moon and what I modeled myself after as a person."

By mid-afternoon, as the temperatures rose, the girls' mother, Doris, sat down in the family kitchen to have a piece of coffee cake with a visiting neighbor while 11-year-old Patti watched cartoons in the den.

Debra decided to take advantage of the break from unpacking to cool off with a quick shower.

While she was in there, the phone rang – and the family's world fell apart.