doesn't get upstaged very often -- but 's brother, , may have done just that at Sunday's game ... where the dude was shirtless and ready to rage.

The Eagles center -- whose season is over after his team lost in the playoffs last week -- was hanging out with T-Swift and a bunch of other friends/family in a private suite at Highmark Stadium as Travis and the Chiefs took on the Buffalo Bills in a huge game.

You could tell JK was ready to party from the get-go, because pretty quickly ... he lost his shirt, and was going bare-chested through much of the game, crushing beers and roaring.

Of course, Tay-Tay was there next to him too ... screaming her head off, all while knocking back drinks with and other gal pals in tow, like . From the looks of the photos of their celebration, they were having a damn good time in there.

Now, at one point when things settled down, some fans caught Jason's attention right below their box -- namely, young girls who apparently had some Swiftie signage they wanted to show off.

Jason saw this, climbed down to them (again, shirtless) and obliged their request.

He picked both girls up and carried took 'em to the window, where they were able to waive hi to Taylor and everyone else inside ... and Jason was just playing everyone's cool uncle.

Pretty sweet moment, we must say ... the guy's totally comfortable, and so are the parents.

As for what Taylor, Travis and everyone else got up to after the game -- where everybody was riding high off of a nail-biting win -- the details are a little unclear about where they went to party, but there's a photo going around showing Trav and co.

posing for a group pic.

The good times keep on rolling for this crew ... next stop, Baltimore. Then, maybe SB!