We have a similar situation in our family, and I would strongly advise against telling your daughter. It would negatively affect her relationship with her mother and her grandmother in a very devastating way. The net result would be that she would realize that her grandmother probably encouraged her mother to kill her brother or sister and almost succeeded in convincing her mother to kill her. That knowledge would be mega-damaging to all concerned!
Pray for your daughter, that she would come to an appreciation for life. Your prayers will do way more than you can ever imagine.
There are several things I’ll never tell her. That is one of them. If God wants her to know, He’ll tell her in His way.