Chris Pratt & Patrick Schwarzenegger Bro Out at Clippers Game

Chris Pratt is throwing off signs his relationship with Katherine Schwarzenegger is super serious ... so serious, he's already broing down with her brother.

Chris and Patrick Schwarzenegger hit up the Raptors-Clippers game (the Clips got clobbered) Tuesday night at Staples Center. Katherine was nowhere in sight, which makes the outing even more interesting.

It's not the first sign Chris and Katherine are showing they're in it for the long haul. He and Katherine hang out all the time with his son, Jack. And, Chris has already met papa Schwarzenegger.

Another good sign for Chris and Katherine -- when "Aint' No Mountain High Enough" came on during karaoke time on the jumbotron ... both Chris and Patrick joined in on the fun.