Why is that? I have masturbated many times with the image of me just doing nasty things with an imaginary sister and it surely adds to the experience. I sadly dont know how it would feel in real life, maybe it would be embarassing and awkward, that is, not sexy awkward like when I am masturbating but just odd awkward. Well, lucky you who can fuck your cousin atleast, my cousin is not that hot but just beeing my cousin makes her so much hotter. Also she is 16 so that would also be an issue, better for me to just wait 2 years, right?
as you said, you don't know thats why you think you would enjoy it, I can't speak for everyone (the world is full of sick freaks) but that is seriously fucked up, I can understand if someone grew up and only met his sister after 15-20 years for some reason, because she won't feel like she is your SISTER you know, but besides that, I think it makes no sense whatsoever to seek legalizing something like this.
it's like legalizing consensual homicide, that is a person agreed with a written legal letter that he wants to be killed by a certain individual and he killed him, how fucked up would legalizing that be?