Deborah Ledford made a guarantee before last night’s game.

“I promise you this,” she said. “My son’s team is going to win.”

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And she was right. Problem was, her son’s team also lost.

Ledford is the mother of the Nets’ Brook Lopez and the Knicks’ Robin Lopez, 7-foot twins who have turned an intracity rivalry into a sibling rivalry. She was in attendance last night at Madison Square Garden as the Knicks defeated the Nets, 108-91.

Ledford, who lives in California, said she has attended every game in which Robin and Brook have faced one another in the NBA. She calls those games the toughest of the season because one has to lose.

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“My dream was always to have them on the same team,” she said. “But having them in the same city is the next-best thing.”

Being in the same city enabled the family to have Thanksgiving dinner together last week for the first time in nearly a decade. But last night, from the all-Lopez jump ball to the final buzzer, they acted like strangers rather than twins who recently shared a turkey.

Brook had 21 points and four rebounds. Robin had seven points and five rebounds. And his team cruised.

“It was a little different than times before,” Robin said. “Past couple of seasons, it’s kind of been the same thing, old bag. But tonight, the Brooklyn/New York thing going on, it kind of felt new. It felt rejuvenated. It was a lot of fun.”

When asked what he thought of his brother’s play, Brook responded simply, “It was fine.”

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It’s easy to tell one from the other now, but when Robin and Brook were toddlers, Ledford color-coded them so people could tell who was who. “I dressed Robin in red, ‘r’ for Robin,” she said, “and Brook in blue, ‘b’ for Brook.”

As they got older, their appearance remained the same, but their playing styles began to differ. Brook developed into the more polished offensive player and Robin became the scrappy rebounder and defender.

“Brook taught himself to dribble at the age of 2 and always had a passion for shooting,” Ledford said. “The other little boys on their teams all wanted to shoot too, and Robin was happy defending for the whole team. He found his role.”

Ledford wanted her children to be well-rounded with diverse interests, so she involved them in a variety of activities. Robin liked to draw, Brook liked to write. Robin played the drums, Brook played the saxophone. Both acted in plays and both read a lot. They remain fanatics of comic books, Disney and Star Wars.

They graduated high school with a nearly identical GPA, both over 3.7, and went to Stanford University, their mother’s alma mater. They spent two seasons in college before entering the NBA in 2008. Separated for the first time, they became opponents, albeit rather identical-looking ones.

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Against one another, Brook entered last night’s game averaging 20.3 points and 7.7 rebounds compared to Robin’s 10.7 points and 5.0 rebounds. But Robin got the win last night, improving his career record against Brook to 6-4. And Robin did so while his mother sat in the stands wearing his jersey.

Favoritism? No. When Brook and Robin play each other, Ledford wears the jersey of the home twin and hat of the visiting twin. “She’s a smart lady. She’s a Rhodes scholar,” Robin said. “I know she’s rooting for both of us. Mostly me, I’m the baby . . . By a minute. I’m still the baby.”

Is that true, Mom?

“All I want is for both of my boys to play well,” Ledford said. “One is going to win, one is going to lose, that’s the way it is. There are many more games in the future.”

And she promises that her son’s team is going to win each one.