Mu Husband And My Sister

I let my husband have regular sex with my sister. She's actually two years older than I am and single and I knew early on that they fancied each other. My husband, and sister, are more highly sexed than I am so it seems a good arrangement. I was once told that it's better to pick your husband's lover. We did try a threesome once but I didn't like watching him actually doing her and of course, cumming in her. He's snipped so no chance of getting her pregnant. It's a mutual arrangement and it suits us all.