5:52 PM PT -- Travis is clearly appreciative of his better half showing up to support him ... after he scored a TD, he seemed to blow a kiss and toss a "hand heart" up toward the suite where Tay's chilling with the fam.

His brother was freakin' stoked too ... though Jason went with a pretty different kind of celebration -- ripping off his shirt and downing a beer while the boos rained down on him!!!

3:17 PM PT -- So far, so good on the all-important "meeting" of the sibling ... Taylor and Jason look pretty comfortable chatting -- which makes you wonder if they've maybe hung out before.

Taylor Swift's landed in Buffalo and Bills Mafia is giving her a hard time ... but the pop star seems to be taking all the jeering in stride.

TS' beau's playing in chilly upstate New York this afternoon, and her arrival at Highmark Stadium received the requisite amount of fanfare ... lots of pictures, video and a little trash talk from the other team.

One fan screams out "Bills by a billion" and Taylor seemingly blows a smooch back to him in response ... pretty dang cool response for the megastar.

Tay's basically a pro at attending Chiefs games at this point ... she's been hitting them up regularly since September and even braved frigid temperatures to watch her man's first playoff game last week -- and seemed totally enthralled with him after.

And, there's another relationship milestone Taylor's reaching tonight ... a public appearance with Jason Kelce, Travis' brother, who plays for the Philadelphia Eagles and was spotted landing in Buffalo earlier Sunday. She's already been spotted with JK's wife Kylie ... so it shouldn't be too long.

As far as we can tell ... Taylor spent a ton of time with Travis' mom, Donna, and even hung out with his dad, Ed, but she and Jason have never been spotted hanging out publicly -- which doesn't mean they haven't had a covert meeting over the last couple weeks.

Now, Chiefs fans might not be so stoked on Taylor's appearance at the game ... remember, she pulled up for the match back in December where the Bills won by just three points -- and Patrick Mahomes had something of a meltdown after the L.

Of course, Taylor didn't cause that loss by any means ... but sports fans are superstitious -- so you just know she's gonna take a lot of flak if Josh Allen's squad can pull out a win.

The game's kicking off here at 3:30 PM PT ... so grab a seat and blast "Cruel Summer" in honor of the hottest couple around!!!