Trae Patton/NBC/NBCU Photo Bank via Getty; Taylor Hill/Getty Kelly Clarkson's ex-husband and former manager Brandon Blackstock is firing back at the lawsuit she filed against him in March.

They share a son named Shelby, and Reba was a long-time stepmom to Blackstock's children from a previous marriage (including Brandon Blackstock ... relationship with actor Rex Linn.

The singer shares two kids with ex Brandon Blackstock: daughter River Rose, 6, and son Remington Alexander, 4 Benjamin VanHoose is a Staff Editor on the Movies team at PEOPLE. He has written ...