For Travis and Jason Kelce, blood is thicker than water ... because the Kansas City Chiefs star is singing on a Philadelphia Eagles Christmas song despite an NFL rivalry.

Here's the deal ... Travis is lending his voice to a track on the upcoming Philadelphia Eagles holiday album, "A Philly Special Christmas Special," singing a duet with his brother Jason.

The Kelce Christmas song is called "Fairytale of Philadelphia" and it's dropping Wednesday ... ahead of next month's full album release.

Travis and Jason are super close, but they play on rival NFL teams ... with their squads meeting in last season's Super Bowl. They play the rematch this week on Monday Night Football.

Frankly, it's kind of strange for Travis to be on a Philadelphia Eagles holiday album ... and some are questioning if the Kelce brothers' relationship is going too far. Sure, they host a weekly podcast together ... but this is a whole different ballgame.

Of course, there's a Taylor Swift angle to this too ... the main reason Travis is being featured on an opposing team's album is because he's dating Taylor.

In a preview for the song, Travis sings "You are handsome" to Jason, who replies, "You are pretty." Travis then sings "You're the king of South Philly" before they launch into song together.

Jason's Eagles teammates Jordan Mailata and Lane Johnson are his partners on the album ... and in the song with Travis they say, "We're going to let a Chief steal the show? Hell no!!!"