I always advise my clients and even my friends about what the best possible outcome could be about taking an action and if there is no positive to simply not do it.
I doubt that you ex-husband's family will ever believe that he was sexually and mentally abusive of my your daughter, as well as abusive of you. So, it id doubtful that your letter will convince them otherwise. I completely understand your desire to want them to see the "truth" of what he has done. But, people are likely to be in a deep state of denial, especially about a family member. You can definitely try. However, understand that your letter will likely not be received and acknowledged for its true intent. They will get angry and upset. They might even file for a restraining order against you for harassment. I have seen this happen before. You can contact them in order to find out his whereabouts to get him to pay child support. However, they will likely not be forthcoming after hearing about the abuse. They have already cut off contact with you, so they have already taken sides. The best way to make sure that your daughter does not think that she did anything wrong is to get her into a good cognitive behavioral therapist. Good luck
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
The key question is what you hope to accomplish by contact your ex-husband's family and telling them "the truth." If you are seeking to establish his whereabouts, just contact them and ask if they can assist in helping you locate him. When there has been a major event such as sexual abuse, I sometime advise clients to work through an intermediary when attempting to reestablish contact with an ex spouse's family. I would also caution that there is some research suggesting that sexual abuse of a child by an adult often occurs when the abusing adult was himself or herself a victim of abuse as a child. Even though it sounds like your daughter may be nearing adulthood, be cautious about encouraging a relationship with the family that may have helped create a sexual abuser.