i totally sympathize with you. i have a friend who was into yaoi/shounen-ai stuff, only her main story that it was from was - Harry Potter... between Harry and Drako. she would look up fan drawings all the time, print them off, carry them around in a notebook, show them to us, wrote her own fan-fic about the two of them, and played rpgs based upon it. my Christian friend and i flat out refused to look at the pictures, listen to her go on about her fan-fic and fan art, but the other people in our group humored her. her excuse - in a squeally fan-girl type voice - "but it's cute" ...
that seems to be a pretty darn common excuse... puppies are cute, kittens are cute, people of the same gender behaving romantically toward each other is not, in any way.
as far as advice for dealing with her... beyond what has already been said... maybe giving her consequences for what she is doing... anytime you find her looking at yaoi stuff or writing it, she gets suspended from using your computer for a period of time, with increasing length for each repeated offense.
and i will pray for you and your sister.