His sister Jyl’s problems “continually chip away at (Julia and Danny’s) marriage.” An insider insists, “Some of their biggest fights have been over Jyl.”

Jyl’s past includes multiple rehab stints and three DUI busts, reports the Enquirer.

“Instead of letting Danny rush to his sister’s side, Julia put her foot down and insisted his place was with her and the kids.” Julia’s even delaying the Moder family’s efforts for an intervention, according to the source. “Julia’s making a bad situation even worse. She wants to handle the situation personally — but not until she’s done filming (her latest flick, ‘Duplicity’).”

“(Jyl’s) always thought Julia stole Danny away from his first wife, Vera, and even once called her ‘the devil’ at a family wedding.”