Sex, Husband And My Sister

I'm not as highly sexed as my husband who I love very much. He's very considerate but I know that he wants more than I do but he's not excessive. If we don't have sex he likes me to wank him off so that he can shoot on my body. Luckily, I don't mind cum over me (but I don't like the taste). Around a year ago I told him that if he had a mistress just for sex and as long as I didn't know who it was I wouldn't mind. He said that he wouldn't do it but I now suspect that he and my single sister are having regular sex. They don't know that I suspect them but I'm afraid to challenge them over it. When we do have sex I can't help but think that his cock has recently been in my sister. Strangely I find it a bit of a turn on. I just live with it, I don't want to lose him over sex.