I do not believe that your sister has no idea that your husband is attracted to her. If you want to have any hope of saving your marriage, then keep your sister away from your husband. Tell her your reason, if you feel that will make it easier, but just do it. Eventually, your sister will have a man of her own. That will make things a lot easier. Until that day comes, do all you can to keep these two away from each other. Talking to your family probably won't do much good at all. By now they probably are well aware of your husband's interest in sister. There is not much that his parents can do about this.

You can always say to your sister, "I'm sorry, but I can't have you coming around my husband because he has gotten too interested in you. I'm not blaming you, but I need to get him out of the habit of taking this wrong interest in you."