My husband, porn, my sister, and me

I have been having issues with my husband and porn. Your article has helped me understand some of his point of view. He actually became addicted to porn over the years and hid it from me even though I didn’t have a HUGE problem with it. (I wasn’t happy about it, but it wasn’t an end-all situation.) However he became emotionally and sexually attracted to my sister, and began fantasizing about her using her pictures for self pleasure and then imagined I was her when we were intimate. They worked together and spent all work day and many work nights virtually with each other. His therapist is telling him it’s normal to fantasize about coworkers etc… however I told my husband a decade ago that if he ever masturbated to her, we were through. So am I being ridiculous? Should I be insulted? I feel violated (I am a multiple rape survivor) if you have time please advise? 🙁