I enjoyed the core concept of this, that is a female protagonist gaining a futa dick through mad science and having fun with her family. I'm certainly in favor of plenty of incest pregnancies and no inbreeding negatives. Could even extend the scope of things a few ways (wider harem, longer timescale, additional generations), but that may be far too much to expect at this time.
Right now, what other posters have already said fits. Work on the initial setup, add some buildup. Show the actual growth of the dick (fast as a few minutes or did it take all five years to reach this size? or any time scale inbetween). Show the accident that caused the drug exposure, have people discuss how it affected them. Show a little of the breakdown of sexual barriers. Make it a steady progression from introducing MC to her sister giving her a handjob. And perhaps tease how things could continue in the future. (is her dick going to get even bigger? will the world get even hornier?)
And I, too, would encourage you to keep working on this. The creation process itself counts as practice in getting better at story creation. Run with it.