Woody Harrelson's brother Brett Harrelson isn't ruling out the possibility they are half-bros with Matthew McConaughey ... and says he might have to start pinching hairs from the famed duo to find out.

We got Brett out in Bev Hills -- seemed to be on a date -- when he told us MC's the one holding up things, because Matthew doesn't want to possibly find out the guy he thought was his father his whole life isn't actually related to him. His dad, James, died in 1992.

Still, Brett says Woody's been working on his longtime pal, trying to convince him that finding out he has new brothers might be worth the overall shock.

BTW, Woody and Brett also have a brother named Jordan.

Brett reminded us his dad and Matthew's mom, Mary Kathlene McCabe, have definitely canoodled, saying their hookup timeline matches up with Matthew's age.

Remember, Matthew's said his mom admitted to him she was more than a friendly face to Woody and Brett's dad back in the day.

All that aside, Woody and Matthew have roots in Texas and no doubt have a resemblance. Plus, they're longtime buds and their families are super close. The "Dallas Buyers Club" star even told Kelly Ripa he's Uncle Matthew to Woody's offspring and his kids call Woody ... you guessed it ... "Uncle Woody."

They already act like family IRL ... and now they're about to costar in the comedic series "Brother from Another Mother." Who needs DNA?!?