I am a mother whose son told me a year ago about the abuse he had experienced. I think you need to consider the questions that have been posed--especially how you will react if your mother doesn't take it well. (I think people sometimes try to disassociate from such shocking news and it takes time for the real reaction.) I didn't react much at first. I was too stunned. Eventually, my son and I started having conversations about what had happened and how he felt. My biggest concern since then has been "What can I do to help him heal?" Your mom sounds as if she might feel the same way. And I think it's important to tell her since you are obviously thinking about it. Keeping it inside may just add to your stress.