My husband is fantasizing on my sister..

This is terribly painful for me as my husband when we were having sex was actually fantasizing on my sister. I couldn’t believe it and still in shock and makes me want to throw up. I don’t know what to do, we’ve been together for 18 yrs and I love him but this is sick.. I don’t know how to process this kind of sickness. I talked to him and he admitted it. Pls advice... I’m so hurt and don’t know what to do...