Cheating Husband

Asalam. I am in need of help. I found out that my husband has been cheating on me for the past 3 years with my own sister. I trusted both people but I am absolutely torn, and don't know what to do.My husband put his hand on the Quran and told me everything and apologized.He says he repents and is truly sorry and for the sake of out 5 year old son, I should not leave him and for Allah's sake I should give him 1 last chance. I cannot tell my parents becuase they are both heart patients and won't be able to handle the disgrace from this. I also cannot talk to my in laws about this becuase they are sick also and my father in law is my dad's brother. I do not want to ruin their relationship. On the other hand, my sister continues to lie and I caught her committing zina with 3 other men too.She says my husband was forcing her to commit haram relation with him, but I'm confused becuase she was with other men too. How do I help her get on the right path? What do I do, I have been praying for Allah to help guide me to the right decision ? Should I leave my husband and tell my parents or stay with him to keep my parents and in laws respect intact? Please help me.