The 39-year-old's father, actor Richard Beckinsale, died of a heart attack in 1979 when Beckinsale was just five years old.
Several years later, the star's mother, actress Judy Loe, married director Roy Battersby.
"It was definitely a shock. I was not that thrilled with it at first. The two youngest boys lived with us. They had red hair and were from a very different sort of background. I was in a private school taking clarinet lessons... And they were in a public school hearing really obscene jokes," she said.
However, despite her initial frustration, Beckinsale insists the boys helped her to be more grounded, adding, "We really rubbed each other's corners off in a great way."