If someone would like to take the idea and run with it. I wonder how many Chapters could be added as I would like for it to be at least 10-20 Ch. in length.
For 10 to 20 chapters, there had better be a LOT of women on that island if they move to a deserted island in the very first chapter! 'D
Though, story composition-wise, there's a lot of ground between 'seeing your Dad dating your girlfriend' and your 'mother and sister agreeing to not only sleep with you, but taking you as their live-in mate'!
If anyone did write such a story, the father and girlfriend and the deserted island would be minor elements, while the majority of the story would be about the sister and mother deciding to abandon the social/sexual norms and not get into trouble.
Typically incest stories fall into two separate categories:
1) Someone sees his sister/mother in the bathroom and they immediately have sex, and spend the rest of the story convincing EVERYONE to have incest with their relatives to, or
2) the sex is slow to develop, as the story focuses on how the otherwise two 'normal' individuals decide to abandon all social conventions and hump like bunnies.
Hint: I tend to write the second kind, and while I enjoy the first, the second takes more time to develop and ends up being more enjoyable by creating a more believable alternate world.