>>57342396because they spend their whole lives and grow up with having an older male (most women are attracted to men older than them, statistically) present in their lives acting as a kind of secondary father-figure role except because they are not the actual father and just a brother they are less involved - this creates an emotional distance/lack of parent-child relationship that usually prevents sexual attraction from forming.
I would guarantee that most of the little sisters that are attracted to their older brothers are ones who didn't have much interaction with their older brother in early life so they subconsciously see them as more of a 'random male stranger who is just there' rather than a sibling - whereas the little sisters who have less of a significant age gap and/or spent a lot of time with their older brother growing up probably don't feel attraction to them (or significantly reduced/controlled attraction to them).
Probably also if they have a very strained/bad relationship with their father and the older brother steps in to fill in the surrogate role of their father figure (like how a lot of bfs do for women with daddy issues); so in essence the brother takes on the role of 'the bf/male giving them the attention their father never did' - an emotionally incestuous relationship probably starts first where the brother and sister are acting like bf and gf in a social/relationship dynamic way before the actual physical attraction forms.
