Incest Survivors Support Group
Incest refers to any sexual activity between closely related persons that is illegal or socially taboo. Consensual adult incest is very rare. Incest between adults and prepubescent or adolescent children is a form of child sexual abuse that has been shown to be one of the most extreme forms of childhood trauma. If you or a loved one is a survivor of incest, join the group...

Regarding the dentist and such, I had to go to the gynecologist recently, and this is a thing for me... when she put my feet in the stirrups, I started sobbing and panicking, but when I told her a little bit about my history, she was very kind, and she helped me through the process. Have you thought about telling your clinicians about your history and asking them to help you through the process?
Have you considered finding a therapist who does specialize in incest ... or at least PTSD? He or she might be better able to help you than your current therapist. If you feel like you can't talk to your therapist about it, that is a problem. You should be able to share what you are feeling and what you have experienced.
Maybe you can look for a new dentist... go into an appointment, explain your history and why this is an issue, and ask him or her if they can be understanding of it? Sometimes we have to look for the right people to be on our team.