Britney Spears' 8-year-old niece is in "extremely serious condition" following an ATV accident Sunday. Maddie is the daughter of the singer's younger sister Jamie Lynn. The little girl was reportedly riding in a Polaris off-road vehicle during a family hunting trip in Kentwood, Louisiana. A source close to the family told Inside Edition that Maddie was underwater for about seven minutes and unconscious. She was airlifted to a nearby hospital.