Amanda Bynes and Jennie Garth pose in the Gold Lounge at the 2007 American Music Awards held at the Nokia Theatre L.A. LIVE on November 18, 2007 in Los Angeles, California. Credit: Charley Gallay/AMA/Getty Images

Amanda Bynes' parents aren't the only ones getting the silent treatment.

The troubled 26-year-old actress has also been ignoring Jennie Garth, who played her sister from 2002 to 2006 on the WB sitcom What I Like About You. Us Weekly spoke to Garth, 40, at ELLE's Women in Hollywood event in Beverly Hills October 15.

"I tried reaching out to her and I haven't been able to reach her," said Garth, a single mother of three. "My heart feels for whatever she is going through and I love her dearly."

Bynes' agent, publicists and lawyer have reportedly jumped ship, while her dentist dad, Rick, and her office manager mom, Lynn, are helpless. "Amanda shut them out and isn't speaking to them," a source said in the October 8 issue of Us Weekly.

The actress hasn't worked since 2010's Easy A (she famously retired then "unretired" from acting via Twitter), and since then, her behavior has become increasingly erratic. Bynes' BMW was impounded September 16 after she drove on a suspended license (following an April DUI arrest and two alleged hit-and-runs). In recent months, she has been spotted muttering to herself in stores on both coasts.

"Amanda is out of control," one insider explained. "It's been slowly going on for years, but now it's getting really bad."