Danger of incest
I wondered about this before but the movie made me wonder again and isn't really too far fetched.
I mean if someone donates sperm there actually IS the possibility of fathering 100 kids(maybe not 500). I don't know if there is a certain limit of pregnancies that can be produced by one donor that is legal. http://www.nytimes.com/2011/09/06/health/06donor.html?pagewanted=all
The possibility that two half-siblings meet as random strangers isn't really impossible. Of course this can also happen with adopted children after they grow up if they didn't know the have a sibling somewhere but the possibility is much higher if you have over 100 half-siblings.
It could easily happen that they have sex with the unknown sibling because there is no way they could ever know.
I wonder how this problem is handled and if it ever happens.