"My sister was my very best friend in the world," Givens, 51, tells PEOPLE. "Her passing was a shock. I had someone describe it to me like losing somebody in a car accident. I was so devastated."
Although Stephanie wasn't feeling well, Givens said she "didn't look sick at all," and when she went to the doctor, they told her it was probably just the flu and to return the next week.
"She went back the following week and was put into the hospital on Thursday, diagnosed on Friday and died on Sunday," Givens painfully recalls. "It all happened so quickly that it's hard for me to even know what happened."
The actress, who stars in the faith-based film God's Not Dead 2, says she got through that extremely challenging time by focusing on being strong for her mother and two sons.
"It was the first I understood when people say you have to be strong for someone else."
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These days, Givens says she honors her younger sister, who was a martial arts expert, "in every way she can" and focuses on the happy times.
"I remember the last Christmas we spent together. She came out to Los Angeles and we went to the Ritz-Carlton. It was beautiful here and we had champagne outside and then went to a Korean spa. It was so much fun."