Fucking my sister's boyfriend, i do better than her

Although it may not seem so, this boy i entered is my sister's formal boyfriend and fiancé. she had no idea what happened that day, but i got what i wanted so badly: to fuck with him. from the first day he introduced me, i wanted him, and i know he wanted me too. once alone she confessed to me that she was bisexual, that she had also had sex with other boys, but not to tell my sister. that made my cock like cement. so one day i couldn't take it anymore and i threw the things straight at him. sitting on the couch while my sister was out we started kissing, undressing and eating each other's cocks. shortly after, my cock was penetrating her tight ass and we fucked until we came. shortly after she left my sister and today we have sporadic relationships without her knowing.