You have an interesting concept, but the story currently lacks depth, even for a story that is supposed to be pure smut. You can use more sensory language to immerse your reader into the scene. You should be describing scenes and actions as if you were intimately familiar to a complete stranger.
If you want to add plot, always ask "why?" Why did the sister really come home early? (Did she want to surprise her kid brother while the parents were away? Something else happen?) Why is he so attracted to her sexually? (Pure physical attraction or were they closer in younger years?) You don't have to reveal everything to the reader, but having those questions answered yourself will help with direction. Who knows, maybe big sister is secretly trying to fulfill a secret family obligation and wants to use her brother to do so.