No memory but lots of nightmares

I have an abusive family and I’ve been sexually assaulted by others as a teen/adult. I have no memories of being assaulted as a child but had a lot of signs, such as wetting the bed up to age 11, being scared of men, and knowing too much about sex.

I’ve been diagnosed with PTSD and often have nightmares. These past few years, though, I’ve started having nightmares frequently of my brother assaulting me.

For those of you with similar experiences, did you ever find out why this was happening? What have your therapists said?