But her mother insisted they both go out and have fun.
When Annie got home at 3:30 that morning, the door was locked. "I was pounding on the door," Annie told ABC in 1994. "And then I noticed his car was gone, so I got very scared."
When police and firemen arrived, they broke down the door and found Anne Scripps Douglas in Annie's bed, bleeding from the head. The wounds were so severe, it was at first thought she had been shot. It was later determined she had been attacked with a hammer, apparently the same one she slept with for protection.
Alex said she immediately suspected Scott had done it. "I knew it from the second they told me that it was him," she said.
Police found Douglas' grey BMW abandoned on the Tappan Zee Bridge, the engine running, a bloody hammer on the front seat. They suspected he had jumped, but no one was sure.
But Annie and Alex were sure -- that Scott had faked his own suicide. They thought he was on the run, and might be coming after them next.
Anne Scripps Douglas died six days later without ever regaining consciousness. Annie and Alex were heartbroken.
"My mother was our best friend," Alex told ABC in 1994. "It's so hard when you lose your mother and your best friend."
The horror was compounded by the realization that their stepfather was the suspect. Their 3-year-old sister, Victoria, had apparently witnessed the gory scene. "She hid under the bed from daddy when he was calling her name, and she said, 'Why did daddy hurt mommy?'" Annie said in 1994. "'Why does mommy look like a monster? Why does mommy have paint all over her face?'"
Alex and Annie had no doubt that Douglas was still at large. "I don't see Scott jumping," Annie said. "I don't think he could take his own life. I think he was a coward."
The Scripps family even offered a $100,000 reward for Douglas' capture. But three months later, the winter ice gave up Scott Douglas' body. He had indeed jumped from the Tappan Zee Bridge sometime around midnight on New Year's Eve. Annie and Alex no longer had to fear for their safety but closure would not come so easily.
"I felt guilty," Annie told ABC in 1994. "I just felt it was my fault at first, too, that I shouldn't have gone out, and that I could have stopped it. Our lives will never be the same again."
Over the years, Annie and big sister Alex tried to get on with their lives. They got married, had children and got divorced. Through the ups and downs, the sisters stayed extraordinarily close.
"We'd talk probably like 10, 15 times a day," Alex told "20/20"'s Deborah Roberts. "We went on vacation together, and if I went on vacation, I called her 10 times a day, and she would call me. And all my friends were like, 'Oh, my gosh, you girls are nuts.'"
For Annie, so close to and so like the vulnerable mother for whom she was named, getting on with life was more of a struggle. "She felt so guilty because she said if she [had] stayed home, she thinks she would have prevented it," Alex said of her sister. "I said, 'No, Annie.' I said 'No, he would have killed you, too.'"
Alex says her sister saw a therapist but to no avail. "She would break down into tears, saying, 'I miss mommy, I miss mommy, I want mommy back' ... almost like she was a little kid again."