Think about it this way:
if your sister was extremely hot, you'd just have been denying your attraction before reading shit like this anyways. people with sisters will swear up and down this could not happen in real life, but their sisters are pretty much shit compared to the ideal you see in manga.
i have a sister, she is not hot, i most definitely do not lust after her even though sister shit is pretty much my favorite type of incest fetish, if i had to choose one(although i prefer non-blood-related, so i guess i need to raise my power level more). i can guarantee you i would think about it if she had my body type of preference, though.