“This movie is a win for the whole family.” So says 15-year-old actress Saniyya Sydney in a promotional clip advertising the new tennis blockbuster King Richard.

Although Sydney’s line was written with cinemagoers in mind, it could also apply to the film’s subjects. Yes, Will Smith occupies the foreground with an Oscar-worthy turn as family patriarch Richard Williams. But we also see plenty of mother Oracene and the three half-sisters who never became tennis pros: Yetunde, Isha and Lyndrea.

Travelling around in a yellow Volkswagen minibus, these older siblings were almost like extra parents for Venus and Serena. As well as collecting tennis balls during daily practice sessions, they also participated in endless card games and staged talent contests. Not just because they were trying to look after their younger sisters, but also because it was fun.

And then, once the two prodigies began to play professional tournaments in their mid-teens, the whole family worked together to insulate them from the worst instincts of American society. Speaking to Telegraph Sport last week, Isha recalled sifting through mountains of mail in order to eliminate all the hateful messages.

“I remember being an undergrad in 1997,” said Isha Price, during a phone interview from Warner Brothers’ offices. “I would come home to Palm Beach in the holidays and find stacks of unopened mail. When I went through it, there would be requests for photos and stuff, but at least a third of it was hate mail.

“It was people calling my sisters ‘monkeys’, or saying that they shouldn’t be playing tennis at all – ‘go play basketball instead’. As a family, we didn’t want them to hear that. So we started telling them, ‘Do not read the papers, do not read your own fan mail, just focus on your game, speak with your racket, show passion in what you do.’”

While Venus and Serena followed this advice to the letter, there was no escaping the racist remarks or boos that would occasionally ring out from the stands. As the elder of the two – and the first African-American great to emerge since Althea Gibson in the 1950s – Venus served as a sort of John the Baptist figure, paving the way for her sister to pile up titles like other young women might collect ear-rings.

“Venus took the hit for a lot of that stuff,” Isha told me. “Which is one of the reasons why she became so stoic on court and showed a lot less emotion. But her doing that allowed Serena to be expressive and emotive, to be her whole self. Serena had more friends on the tour. She was a lot freer because Venus took the hit.”