I just found out that my husband had an affair with my sister for almost 3 years, what to do?

My husband and I have only been married for 3 1/2 years we have two small children he says hes really sorry and hell never do it again. I questioned his feelings for her and vice versa and he says he has no more feelings for her and he really wants it to work with me. Shes also married and she used to be the best sister in the world. We were really close, I am devestated and i got a double dose of betrayal and hurt. I just need to know their true feelings and if i'm wasting my time trying to make it work. he says he loves me now more than ever because he sees how much i love him to be able to give him a second chance. I'm just worried about the length of the affair and why it happened in the first place. please guys, no slamming on me, i've been through enough and i'd appreciate sincere, helpful advice if anyone has been through this. Thank you.