I don't think you can over-share in this situation. Silence is what keeps these predators in business. The more it's discussed how unacceptable it is, the fewer people will help to enable it or cover it up. I'm not saying that is what your aunt is doing, but it's better to be overprotective of children and hurt one person's feelings, than to spare her feelings and potentially have more children hurt.
That said, I'm not opposed to taking GalaGirl's approach of seeing if she's okay first. If she says no, let your mother know so she can be the support person in that case as you said she would. If she says yes, you are good to go.