UPDATE

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 actually hung out before.

'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.

“bills by a billion” *BLOWS KISS* I LOVE HER LMAO

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 and to watch her man's first playoff game last week -- and seemed totally after.

And, there's another relationship milestone Taylor's reaching tonight .

.. a public appearance with , ' 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 -- and had 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 '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!!!