Amanda Bynes - Siblings Draw Battle Lines ... Our Dad Is NOT An Abuser

The brother and sister of Amanda Bynes have lashed out at her for attacking their dad and accusing him of physical and sexual abuse.

Jillian O'Keefe and Tommy Bynes tell TMZ through their lawyer, Tamar Arminak, "We are disturbed beyond words that Amanda would come up with such a fabrication as a way to avoid getting much needed help or treatment."

Amanda went on the warpath against her dad Friday after she learned her parents were coming to NYC to get her into a mental health facility, tweeting her dad had sexually and physically abused her ... something her dad vehemently denies.

Jillian and Tommy are right behind their father, calling their parents "the most wonderful parents who have ever lived."