I want him but i know its wrong, what to do?

my step sister used to date this guy and she really liked him. then they both just rowed and it ended but they still stayed friends and all hung around in the same group and they would still end up sleeping together after. but one night after weeks of flirting i ended up sleeping with this guy. my step-sister said she didn't mind as she had another boyfriend at the time and she was really into him. so me and this guy carried on flirting and he seemed to really like me. then i went away for a bit and when i came back my step sister was single again and was back to liking this guy. she thinks he really likes her when im not sure if he really does i think he just looks as it as a shag. but now i feel that they're are back to liking each other and im completely forgotten. i have such desire for him again and cant get him off my mind. i want him so bad but i know its wrong because hes my step sisters ex and she really likes him still. but i can sense he wants me too as he flirts with me. but im just so confused because i do really want him but i dont want to hurt my step sister. what should i do? stay away from him completely or just enjoy the flirt and the thought of him??