How Molly Shannon's Loss of Her Mom and Sister in Tragic Car Crash Inspired Her 'SNL' Character

Molly Shannon caused peals of laughter with the portrayal of her Saturday Night Live character Mary Katherine Gallagher, who loved to smell her own armpits and proudly called herself "Superstar!"

But behind the comedic act was grief the star still felt decades after her mother and baby sister were killed in a tragic car accident.

In an exclusive excerpt from Shannon's new memoir, Hello, Molly!, featured in this week's issue of PEOPLE, she remembers those haunting first days after the crash — and how she and her widowed father, who struggled with alcoholism, moved forward.

"I'm a lot like Mary Katherine Gallagher. I'm a survivor," The Other Two star, 57, writes in Hello, Molly!, which will be published by Ecco on April 12.

Shannon was just 4 years old on the June night in 1969, when the life of her mom, 3-year-old sister Katie, and cousin Fran ended. Her dad, who had been drinking, was driving the family home from a daylong party when he sideswiped a car on the highway and then swerved into a steel light pole.

Shannon, her dad and her sister Mary, 6, survived, but they carried around the physical and emotional scars for years after.

"The car was mangled badly on impact. A man passing the scene stopped. My mother was lying on the ground beside our car and she asked him, 'Where are my girls?' " Shannon writes in Hello, Molly!, recalling the accident. "She wanted to gather her three little girls and she couldn't. Her heart must have broken in that moment. And those were her final words.

"My baby sister, Katie, and cousin Fran were killed instantly," Shannon continues. "Since Mary and I were in the very back of the station wagon, we just had a concussion and a broken arm, respectively. Katie was buried in the wreckage."

In the excerpt, Shannon recalls the scary days that followed. She remembers getting up from her hospital bed and getting dressed so she could go see her mom and sister. A relative explained that they were both in heaven, so she couldn't visit them. "I didn't know what death was," the actress writes. "I just wondered, 'Why did Mommy and Katie leave without me? Maybe I'm bad.' "

Shannon's father was badly injured in the accident and had to relearn how to walk. He struggled with feelings of guilt and the challenge of single-parenting two young daughters.

Being raised by a widowed father resulted in a unique childhood. In her memoir, Shannon writes that there was "no pressure to act ladylike."

"I wore whatever I wanted," Shannon continues. "When I had holes in my Keds, my dad said, 'Good, that shows you have character.' It gave me so much freedom later as a performer."

While growing up, she also learned that she loved to make people laugh. In 1983, Shannon enrolled at New York University, where she studied drama and waited tables to pay for tuition. It was in her college days, during an improv audition in 1986, that Shannon first donned the naughty Catholic-school girl persona of Mary Katherine Gallagher.

"I was doing the character as a very nervous person — an exaggerated version of myself and how I felt," she writes. Audience members loved the show. Fellow classmates started telling Shannon, who was focused on dramatic acting at the time, that she should be on Saturday Night Live.

Less than a decade later, Shannon did just that when she made her SNL debut in 1995. She ended her Mary Katherine Gallagher sketch with an impromptu final lunge and said, "Superstar!" Shannon wanted to make her friends laugh — not knowing that it was a move she'd become famous for.

"Mary Katherine wants to be a star and she wants to be seen and she wants her mom to come back from the dead," writes Shannon, who says she still felt an "ache" in her heart despite her rising success as a comedian actress because she missed her mom.

"The character is a survivor... an adult child of an alcoholic. A girl who trips. But gets back up. It's an emotional character. I wrote from my heart," she explains.

Though Shannon still misses her mom and sister, the loss heightens her own experiences as a mother, filling her with love and gratitude. (Shannon shares Stella, 18, and Nolan, 17, with husband Fritz Chesnut, 49, a painter.)

"I don't take any of it for granted," the star writes in Hello, Molly! "Some of the stuff that people complain about, as far as parenting goes, I can't relate to. I just think, They are alive!"

Shannon says she also felt a swell of gratitude when she got to say a final goodbye to her dad, Jim Shannon, who died in 2002 at age 72 after a two-year battle with prostate cancer. (The year before, the actress had a positive conversation with her dad about his sexuality and he came out as gay or bisexual — a fact he wasn't able to openly embrace before.)

"I held his hands in the hospital room. I felt incredibly honored to be there while he was dying," she writes. "I had lost my mother so abruptly. I treasured the good ending with my dad. That the rug was not pulled out from under me so suddenly. That I had time to say goodbye."

Hello, Molly! will be available on April 12.