Hope, I am a mum, I can tell you it is the hardest thing that can ever happen to a mother. And you know what is worse, there is no support for the mother - so she will wallow, and grope all on her own in the dark. Everybody is different, some strong and some less strong and we process things differently. It is not too late for healing for both of you - she can seek therapy.
I completely feel you on this one. I joined this forum a few weeks after my son was charged with molestation of my youngest. There are even fewer resources for parents of siblings in this respect. And it tears me apart each and every day. The only thing that keeps me going is knowing that both of my boys are getting the resources and help they need...I don't feel I am yet, but they are and that's encouraging.
Having been a survivor in my own right from SA in college, it brought up all of those feelings again and each day is a struggle not to let it completely unravel my life. I support both of my sons, they are both survivors (the eldest from SA by his bio mom, the youngest a survivor of the eldest) and am very blessed to have friends/family in my life who have worked with SA survivors professionally. But even with that support, it is so very difficult to move onward toward healing in my own heart.