Since we were kids my sister always wants what I have. I knew she was always after my boyfriends so this time I let her have him, not for her only for my boyfriend. He's a great guy and I knew he wanted to try a threesome with me and my sister so I let him have this gift. We all enjoyed it but I've made him promise that he could do it again only if I was present, he's forbidden to fuck my sister behind my back.