My husband had sex with my son's 17 year old sister.

My husband (M44) and I (F43) have a 17 year old son. My husband has a daughter (F17) from another relationship who currently lives with us. They have always gotten along fine but my daughter in law has started to get distant and quiet since a week ago and started wearing baggy clothes to cover herself. At first we thought she was just being a teenager but yesterday we found text messages from her and my husband in which she has asked my husband to stop and her teacher has told her that she doesn't have to spend time with her family if she doesn't want to. I confronted my husband and he broke down and cried and told me that he had sex with her multiple times and that it was a mistake. He has begged me to not call CPS and to not throw him out. What should I do in this situation?