((((((((((( Amanda ))))))))))))))))
Were these letters handwritten? Or do they contain a signature that is handwritten? I'm simply concerned that he will claim that they are not written by him.
If you do not feel that it would be healthy for you to go through this emotional upheaval again, simply have the letters photocopied and give copies to your aunt and uncle and let them do with them what they wish. You could also include a statement of your own giving pertinent information along with your own testimony of the emotional damage this man did.
If you do feel that being personally involved in the confrontation with him would be beneficial to you and the benefits would outweigh the stress of contronting him, then by all means do so. It will most likely be your last opportunity to do this and it will benefit both you and your mother greatly.
I wish a beneficial outcome no matter which way you decide to go on this.