Hayden Panettiere‘s ex Brian Hickerson is jumping into the actress’ bitter family feud … blasting Hayden’s estranged mother after she publicly questioned Hayden opening up about her sexuality in her new memoir.

Brian sat down with TMZ’s Charlie Neffand reacted to recent comments by Hayden’s mom, Lesley Vogel … the former momager cast doubt as to why Hayden was discussing being bisexual now, suggesting the timing had more to do with promoting her new book.

When Charlie asked Brian if he knew Hayden was attracted to women, he kept it simple — “Yeah. Hayden is into beautiful people.”

But things escalated quickly when Brian turned his attention to Lesley. He didn’t hold back … saying, “Hayden’s mother is by far one of the worst people I’ve ever met in my life … I just can’t with her. She’s terrible.”

The comments add another messy layer to the ongoing drama surrounding Hayden’s upcoming memoir, “This Is Me: A Reckoning,” which has already sparked headlines over her addiction struggles, family trauma, and past abusive relationships — including her tumultuous coupling with Brian.

As we reported … Hayden recently revealed she felt pressured to keep parts of her identity private while growing up in Hollywood under her mother’s management. Meanwhile, Lesley is accusingher daughter of giving her “20 years of trauma” … suggesting the drama is more about book sales than healing wounds.