The linebackers' father, Marvin, is staying in California and away from the anxiety of the moment.
The first time he watched his boys play each other on the football field, he had a heart attack that night. That was in October 2011, when Mychal's Cal Bears visited Eric's UCLA Bruins. Marvin had been experiencing discomfort for weeks, but when he got back to his hotel room, he felt as though an elephant was sitting on his chest.
"I couldn't get rid of the pain," he said.
A proud former Bruin himself -- he played running back for UCLA in the early 1970s -- Marvin told the ambulance driver, "Anywhere other than the UCLA Medical Center, I'm gonna get out and walk!" He had two stents put in and was hospitalized for a week.
"That was scary. Right after the game," said Mychal, the family's Eagles representative. "He gets too excited, man."
In an odd twist of timing, he now needs to have those stents replaced as his sons vie for a trip to the Super Bowl. Marvin said he will undergo surgery in about two weeks, and felt it best to watch the game from his living room instead of raucous Lincoln Financial Field. At home, he can simply shut the television off if he's getting too worked up or doesn't like how it's going.
Their mother, Yvonne Thagon, made the trip along with her daughter, Danielle. To show equal support for the boys, they had T-shirts made up that represent both.
Yvonne is staying cool but knows the reality that awaits.
"I already know somebody is going to win and somebody is going to lose, and it's not going to be pretty," she said. "I'm OK, I just know that one of them is going to lose and that's sad to me. It's sad that I have to see that emotion on my child's face -- not necessarily that I'm going to see it, but that they're going to experience it."
Mychal, 27, is two years Eric's senior. He was drafted in the second round by the Eagles in 2012 and has had an up-and-down six seasons in Philly. Athletic and instinctual, he has regained a more prominent role after a couple of years of receiving limited snaps and finished the regular season with 77 tackles, two sacks and six passes defensed on the league's fourth-ranked defense.
Eric, more of a cerebral player, was drafted in the second round as well, in 2015. He has averaged more than 100 tackles a season since joining the Vikings and is at the center of the best statistical defense in football.
They played each other once before in the pros -- a 21-10 Eagles victory in October 2016. This time around is different. They haven't communicated aside from a brief conversation to figure out the ticket situation shortly after the Vikings' dramatic win over the New Orleans Saints. They are trying to make the other a "faceless opponent," as Mychal put it. But it's no easy task.
"I've tried not to think about it just because it's ... I've lived in a room with this kid for 17 years, you know what I mean?" Mychal said. "And we've lived pretty much the same lives on opposite sides of our state, and now we're on different teams and we're in the same scenario playing the same position. ... It's unreal and it's an opportunity of a lifetime and it's something that we will remember forever."
Said Eric: "It's pretty crazy. Crazy we're both in this position."
Football has been the center of this family's universe, serving as the gravitational pull that has helped keep things together. There have certainly been times when pieces have gone adrift.
Marvin played in the Canadian Football League following his days at UCLA and even spent a summer on the Eagles' training-camp roster, but at some point he lost his way. He was addicted to crack cocaine for much of the boys' childhood, putting him on a dark path he regrettably brought his sons down on occasion.
"I would take my kids with me to the drug places, if you can believe that. Every time I think about that it makes my heart hurt. I took them some places they never should have been," Marvin told Philadelphia Magazine in 2014.
"At times I would go with my dad and God knows what he would be doing, but I would be swimming in canals, walking the canal line and just roaming all throughout Fresno, California, all by my lonesome," Mychal said. "Me being a young kid and with other kids who I didn't really know because we would be in some place in some area where other kids' parents are doing the same things. Very young children -- 5, 6, 7 years old -- doing whatever without any parental supervision."
The responsibility fell largely to Yvonne to raise and provide for her children. She worked endless shifts as a pharmacy technician to make ends meet, and watched her sons' games from the snack bar to raise money for a booster club to help with equipment costs for the children on the team.
Marvin eventually got sober. Mychal credits football for providing the common ground to help them reconnect.
The relationship has endured over the years. Marvin has spoken with both Eric and Mychal this week, he said, even helping to facilitate the trash-talking.
"Mike said, 'Dad, if I have to run across him, I'm going to bust him up!' Two days later I talked to Eric and he said, 'Mychal better not get in my way!' " Marvin said with a laugh.
There will be plenty of nerves Sunday -- running from California to Philly -- but all in all, the family members consider themselves fortunate.
"We're truly blessed, for everything to happen the way it happened and for us to be where we're at now, it's crazy what this game has done for our family," Mychal said, "and how it has brought us together."