"Our thoughts and prayers go out to the Stoudemire family for their loss," the Knicks said on Twitter.
The Florida Highway Patrol said Hazell Stoudemire, 35, was driving a car that struck the rear of a tractor-trailer rig on U.S. 27 at Lake Wales, ESPN reported.
The Knicks said Amare Stoudemire would be in Florida with his family and it was unclear when he would return to the team.
"I know he's very close to his brother so my heart's out for him," Coach Mike D'Antoni said. "He cherishes his family, he does a great job of maintaining all the relationships ...This is just a trying moment for him."
Amare Stoudemire grew up in the Lake Wales area, ESPN said.