Play video content EXCLUSIVE

Jermaine Jackson couldn't BELIEVE it when he heard his nephew Taj's claims that a family member had molested him as a child -- telling TMZ, "I've never heard that. I've never ever heard that."

As we reported, Taj Jackson -- one of Tito's sons -- made the announcement on Twitter today, claiming an uncle on his mother's side of the family sexually abused him as a child ... and Michael Jackson counseled him through the trauma.

Taj says he made the announcement in an attempt to discredit Wade Robson's abuse allegations against MJ, claiming, "That is how I KNOW Wade is lying. Because I AM a survivor."

Taj added, MJ "was a support system for me and my mom" during the abuse ordeal.