Ben Liebenberg/AP Photo

If you take a close look at the sidelines tonight during the NFL game between Baltimore and San Francisco, you may just do a double take.

On one side is Jim Harbaugh, head coach of the 49ers and on the other is John Harbaugh, head coach of the Ravens.  He’s Jim’s big brother.

It’s the first time in NFL history that two brothers will compete against each other as head coaches.

“I really, really, really want to beat him!” said John Harbaugh.

Now imagine you’re Jack Harbaugh, the proud father of these two successful football coaches and a former college coach in your own right.

Jack Harbaugh has a lot to be thankful for this Thanksgiving, but there’s one difficult question — who do you root for?

“Well when he talks to me he says me,” said Jim Harbaugh.  “I’m sure when he talks to John he says John.”

“I have no idea,” said Jack Harbaugh.  “Both teams are going to play tremendous games, play with emotion, play with enthusiasm,” adding that he and his wife plan to “get as far away from the stadium as we possibly can.”

Harbaugh isn’t the first sports dad to face divided loyalties.  Competitive sports have long had sibling rivalries.

In the 1984 winter Olympics in Sarajevo, Phil Mahre edged out his twin brother Steve in the slalom by .21 seconds.  Phil took the gold, Steve the silver.

In tennis, the Williams sisters, Venus and Serena, have played against each other 23 times.

On the gridiron, the Manning brothers, Peyton and Eli, are starting quarterbacks for rival teams.

So the Harbaughs are in good company, even if tonight’s game is a first.

The last time these two brothers competed against each other was back in high school, when they faced off in an American Legion baseball game.

They grew up sharing a bedroom and were constantly competing.

“We drew tape across the room at one point,” John Harbaugh said. “Like a line in the sand — don’t cross this. So of course it gets crossed immediately.”

They learned football from their dad who coached for more than 40 years. Even today they still send him game tape to critique.

“Every Tuesday the FedEx man comes down the street … he’s got his two packets. He’s got his Ravens pack and he’s got his San Francisco 49ers pack,” Jack Harbaugh said.

Tonight, Jack Harbaugh and his wife are in Baltimore, but too nervous to watch in the stadium.

“We want to get a picture,” he said. “Once we get the picture we’re going to go to John’s house.”

The 49ers’ Jim Harbaugh is trying not to make tonight’s game too personal.  But that’s tough, under the circumstances.

“Next to my wife and my dad, John’s my best friend,” he said.  “But in this game he’s just gonna be someone we want to beat.”

His brother John said: “It’s gonna be a moment. It’s gonna be a moment.  Full of love and pride and respect.”

Win or lose, for the Harbaugh family, tonight’s game is a Thanksgiving game to treasure.

ESPN contributed to this report.