Credit: Denise Truscello/WireImage; Jason Merritt/Getty

Guess Brandi Glanville's not a fan of Take Your Future Stepmom to School Day.

Glanville fumed after LeAnn Rimes -- controversial fiancee of Glanville's ex-husband Eddie Cibrian -- apparently shared her vocal talents with the class of Mason, Glanville's 7-year-old son with Cibrian.

On Tuesday, former model Glanville sniped to Rimes, 28, on Twitter: "I told Eddie to please tell you that I think it is highly inappropriate for you to sing in my son's class on Friday. Boundaries!!!"

Over the summer, actor Cibrian, 37, reportedly moved into Rimes' Malibu pad over the summer with sons Mason and Jake, 3. (The couple's romance famously began as an extramarital affair on a Lifetime movie set.)

Glanville added angrily, "It's Mason's week share his family with the class. It isn't your place. Let Eddie read a script. You are not his parent. RESPECT!"