This according to Gisele ... who took part in the summer "fun" -- where Tom famously got so competitive in the children's game and chucked a fastball at his own mother!!

Brady's supermodel wife went on "The Tonight Show Starring Jimmy Fallon" on Thursday to explain exactly how savage football's G.O.A.T. and his fam get when they square up on the court ... and yeah, she says it's as aggressive as it looked.

"They're pretty strong," Gisele said of Brady's sisters. "They were softball players. They're intense!! I was scared ..."

Gisele says the rubber-ball violence was eye-opening ... especially compared to her family's game nights -- where they played sports AS A FREAKIN' TEAM!!

Don't get it twisted ... Gisele found a way to have fun -- she's just issuing a warning for those underprepared for Brady games.

"If they invite you to play this game with the ball, make sure that you're ready for it."