Is it normal or weird??should i worry about this-or am i overeacting???

im concerned about my brother-hes 12 years old and partial autistic-he attends a school for disabled children and he does communicate preety well-the thing is he is going through puberty-he steals my daughters diapers and masturbates in them-even the dirty ones. i have tried to talk to my mother about this but se just looks the other way,another thing she treats him like a 5 year old child even though he has shown interest in his own age activities and he seems to be growing up-i have tried to tell her that he is 12 and not 5 anymore-she buys him toys for lil boys that are 5-8 yearsold-well the thing is -i dont even feel comfortable with my daughters around him-when i give my daughter a bath he watches and he trys to look at her private area-he trys to be around me when i change her-do you think my brother will turn out to be a child molester-do you think he understands what hes doing-i even went so far to tell him to knock it off-last summer he grabbed my5 year old and held her down