I had DID (I'm integrated now) but I always knew I was sexually abused. It was not something that could be hidden. I had emergency room visits. There are medical records. The responsible adults in my life were very aware and talked about it. No one chose to do anything about it because I was a source of revenue. These were the days when there were no mandated reporters and this "sort of thing" was considered a someone else's problem.

Whether your therapist is accurate about you being abused, there is no way to know. If it happened you need to deal with it. If it didn't happen it will mess with your head and family relationships for the rest of your life. Be cautious.