'Family Emergency:' Britney Spears Cancels Rehearsals, Rushes To Niece's Side After Horrific Accident

As Jamie Lynn Spears daughter Maddie continues to fight for her life, the country singer's superstar sister Britney has dropped everything to rush to her niece's side, RadarOnline.com can exclusively reveal.

"Britney has cancelled all rehearsals until further notice," said a source close to the 36-year-old, referring to her "Piece of Me" residency in Las Vegas. (The performances resume on March 24.) "She said she had to attend to a family emergency."

With her schedule cleared, the mom of Preston, 11, and Jayden, 10, has left Los Angeles for her native Louisiana, where the little girl is in critical condition at a New Orleans hospital following an ATV accident.

"Britney's dancers are saying that she has already left to be with her sister and niece," the source added.

Although the star has been tight-lipped about the incident, she did reach out to her fans and friends on Instagram yesterday and ask for "all the wishes and prayers for my niece ."

Last night, the Tangipahoa Parish Sherriff's Office revealed new details about the incident. According to officials, Maddie was driving an ATV when she veered off course and became submerged in water. A horrified Jamie Lynn, 25, and husband Jamie Watson, 34, struggled to save her.

An ambulance arrived on the scene within minutes, and authorities were able to rescue the little girl.

"It's hard to say if she's going to be okay at this stage," family friend William Parson told RadarOnline.com yesterday. "We are all praying she gets better."