I would play the beautiful younger sister, a girl whose love for her big bro or sis gradually developed into something more perverse as the two of them got older. One evening, unable to hide her desires any longer, the girl ruthlessly teases and flirts her way into her sibling's pants and the two finally give in to passion, heedless of any consequences. So, that's the skeleton of the plot anyway. Pretty simple.
I'm very open to suggestions and requests regarding more specific plot/setting elements, but I was thinking that the older sibling would be old enough to be living on his/her own now, maybe around 20, while my character would be a few years younger, like 16-18. The events would probably take place at the older sibling's apartment, with the younger sister going over there to visit him/her and spend a day or two after having not seen each other for a while. I'm pretty much equally open to the older sibling being male or female (futa), but one specific request is that they have a rather large cock, like 8"+.
I was sort of assuming a modern setting, but we could just as well set it in a fantasy or maybe even sci-fi setting and include all kinds of fantastic elements, like the two being some non-human race and so on.