John McEnroe can’t grasp how Venus and Serena Williams get along so well. “I do have a brother [Patrick] that played professional tennis, so I understand at least to a small degree what Venus, as the older sister, was going through,” the court legend told the crowd at a screening of “Venus and Serena” at IFC Center on Thursday. “The way she has treated Serena all these years and been behind her through thick and thin is remarkable because I can guarantee you I would have dusted my brother off and told him to take a friggin’ hike if he’d started beating me. The little brother in my family doesn’t beat the older brother, and that will never happen as long as I live.” Just to hammer home the point, he added, “I never did lose to Patrick.” Before the screening, guests had cocktails hosted by Graydon Carter and Anna Wintour at the Beatrice Inn. After the movie, out Friday, guests including filmmakers Maiken Baird and Michelle Major, Wyclef Jean, Eliot Spitzer headed to La Villette.