My step daughter said she lied about my son trying to rape her!

My 15 year old son was sentenced to a mental hospital for trying to rape his 11 year old step sister. In that hospital we found out was sexually abused by his paternal grandparents. We thought it was true because since he was 8 he has been sexually aggressive (when the abuse started). He is drugged out on medication and was diagnosed with dissociative amnesia because he doesn't remember trying to rape her along with other diagnose. They were starting him on shock therapy Monday. She told me her mom beats her and after a fight me and her mom got into her mom said she had to. I'm not mad at her. She's 11, not 16 with an abusive mom and was crying at how sorry she is. I am jyst in tears. We have to have her say this to the court to get him out of them. I can't believe.... My son is a medication zombie. And for NOTHING.

Edit: She had said he TRIED to rape her. I believed her because of my sons sexual aggression, which he was therapy for. He said he didn't do it, but has mental health issues which is why he was sentenced to a mental hospital. After being told he dud do it so many times, he started to believe it and just thunk he didn't remember. So idek how I'm gonna explain this to him.... And her mother is nuts! Me and her had a fight and according yet my step daughter she was afraid of more abuse if she didn't do what she said.