>Both parties get the benefit of pure, deep familial love, friendship bonds, and then sexual bonds on top of it
>Both parties get experience with relationships in a safe environment
>Both parties get romantic and sexual experience in a safe environment
>Both parties get hands-on experience with the opposite sex's anatomy and physiology, in a safe environment
>Both parties get experience working out ridiculously complicated emotional and social experiences in a safe environment
>Both parties get trial runs of romantic exchanges, both social and physical, in an environment where the other party won't get so assblasted that they leave
>Onee-san gets experience being dominant in a relationship, and gets to experience what control and authority feel like, which makes her more responsible and higher-functioning in life
>Otouto gets to experience being a subordinate without feeling emasculated, which makes him more comfortable not always being the dominant and moving force, which makes for a more stable and higher-functioning life
>Otouto gets to have his first romantic experiences in a completely safe environment of pure love
>The power relationship is defined, but not so extreme as to be predatory
>There's less risk for pregnancy, because Otouto's sperm production won't be up to snuff, and he may not even be capable of producing some of the secondary secretions that allow sperm to not die horribly in the vaginal ecosystem
>If pregnancy occurs anyways, there's less pressure against abortion, meaning less chance for an underage birth that basically tanks a families' future, incest or not
All Onee-sans should /ss/ their Otoutos.
