Well, I'd start by confronting your sister about it. Does her husband know? If not, tell him. He needs to know your pain and betrayal too because you are not in this alone in this. Then I would put your husband in the dog house and let him know, in no uncertain terms, that you are not okay with this and that there will be consequences for his actions. What you decide those consequences should be is up to you (i.e. divorce him, separate from him by making him move out for a period of time, let him sleep on the couch for the next 6 months, etc.). If you wanted revenge, I would say see if your sister's husband was interested in cheating on her with you, but that's doesn't usually solve anything except to level the playing field. If that sounds okay to you and you want your sister and husband to feel your pain of betrayal, then go for it. Alternatively, you could find someone else to sleep with and rub your husbands nose in it for a while (i.e. get dressed up in very sexy clothes, go out to clubs, let someone take you home, and then return back home smelling like sex). Then tell your husband that you can get any man to sleep with you whenever you want so you have no shortage of men to satisfy you. However, if he ever even looks at another woman again, you will kick his ass out and find someone else. Alternatively, you could wallow in self pity, feeling betrayed and hurt, and just accept the fact that your husband AND your sister were laughing at you behind your back while they were enjoying sex all the while.