Quoting ~Elliott's.Mommy~:" My 12 year old sister has decised she is bisexual because shes afraid of men. I told my mom because my ... [snip!] ... only she knows her reasons. I told her I support her and love her the same. It made me sad that our own mom doesnt support her"
I think many, many girls her age have feelings for both sexes. and, at that age, she may be overwhelmed at the idea of "boys," especially since the trauma she has had. if she isnt in counseling, it may help. the counseling should be for the trauma and NOT for her sexuality.
Tell your mom to figure out how to give motherly advice and to not bring God in as a substitute for comfort and love and rational explanations of things.