You should not focus on what your ex or anyone else is saying about you or your daughter. Keep your daughter safe, if you think she is being put into an unsafe environment while with your ex, contact a child custody lawyer, you can have a temporary custody arrangement put into place in light of the recent discover of your ex's father. Support your daughter, if she wants to talk to you, listen and hold her, do whatever will make her feel safe. It is good that she is seeing a counselor, but stay with one, she will need to find confidence and trust in the counselor to be able to speak the full truth to them; it will be difficult for her to find this if she is bounced around counselors. Sit down with the counslor and your daughter and make sure she fully understands the magnitute of what happened to her and what the steps are to keeping her safe for now and forever.
People will believe her and people will think she is a liar, you have to tune the comments out and focus on your daughter's well being.
I know it must be difficult with your ex not believing his own daughter, obviously he is in denial, the man that raised him and kept him safe is now being said to have done horrible things.
Just support your daughter, focus on her.