Justin Holiday, the Knicks’ new shooting guard, spent late nights at West Point FaceTiming with a newborn baby girl.

It was not his baby, but his first niece — the daughter of brother Jrue Holiday, the New Orleans Pelicans point guard. The birth became national news because Holiday’s wife, Lauren, a former soccer star for the U.S. national team, was recently diagnosed with a brain tumor.

Lauren couldn’t have brain surgery until after the baby’s birth, and she is now slated to be operated on in five weeks. Jrue has taken a leave of absence from the Pelicans to care for the newborn.

Justin Holiday, obtained from the Bulls in the Derrick Rose trade, is 14 months older than Jrue.

“He gets the night shift a lot so I’ve been FaceTiming, seeing the baby’s face changing,’’ Justin told The Post. “These days you have to have FaceTime. It’s like looking at my brother as a baby. The resemblance is amazing.’’

Justin is close with the former Lauren Cheney, who won two Olympic gold medals with Team USA.

“Even if she wasn’t my brother’s wife, it’d be a tough thing to hear,’’ Holiday said about learning of her diagnosis. “It’s family — a touchy situation. She’s a tough girl and been through a lot already. I’m very confident she’ll get through this as well.’’

No one knows when Jrue will return to the Pelicans. The baby is 12 days old.

“His main focus is to be there to support his wife, take care of his child,’’ Justin said. “I don’t think basketball is coming across his mind right now. It’s not his main focus and shouldn’t be. I also think his team and everybody else supports his decision to do this, which has been a good thing. People were nagging him about it, but everybody’s been supportive. He’s handling his business as a father and husband.’’

Holiday, a rangy 6-foot-6, had a solid training camp as part of a second unit that excelled in Jeff Hornacek’s estimation.

“It was a good experience,’’ Holiday said. “We’re starting to figure out what each of our roles are. For sure, we know we have a team that will get after it. We’re hungry. I’m my biggest critic and have a long way to go but at the same time I’ve always said my effort will always be 100 percent. I feel I did that this whole camp.’’

Noting a lack of injuries, Hornacek said the only player he held out of scrimmages was Carmelo Anthony, but only to rest him.

“Melo’s played all summer — the Olympics,’’ Hornacek said. “We’re hoping it’s a long season for him.’’

Center Marshall Plumlee injured his thumb but didn’t miss time. Plumlee, the Duke grad who joined the Army reserves, was asked about pacifist Joakim Noah being uncomfortable at West Point: “I respect everyone’s beliefs,’’ Plumlee said. “They may not necessarily be my own. But that doesn’t give me the right to dismiss them. I hold Joakim Noah in very high regard and I’m really grateful to have him as a teammate.”