>>10784274 why you do this?
Because it's true, you all know it, and someone's got to say it.
And while we're at it, consider the little sister characters also prevalent in manga. The reason for their popularity is, that for most of us geeks, our sisters have been the single most intimate female contact of our lives. The only girl we've really played or had fun with ever.
Additionally, assuming a real little sister, when living a tightly-knit house, the girl with no further recollection of the world at large will hold her brother in greater respect thatn could really be attributed. Your little sisters have been the only girls who've ever looked up to you, and in the sexually deprieved mentality of a common geek, this easily becomes confused with interest beyond simple inexperience and family ties.
Just think about your personal lives. When your little sisters grew breasts, can't you admit to not at least once have looked at them, thinking "what if", and when they first showed up with their boyfriends, didn't you feel that something died inside of you?