Treatment of younger sisters (and older ones, in this case) is country-dependent: Korean manwha--ordinary sibling interaction, she's probably the good kid, and the MC tries to look out for her; Chinese manhua--Sister? What's that? Is it food? Japanese manga--either she or the MC is desperate to bang the other, and most of the sister's lines will be on this topic alone. Female MCs are pretty much always only-children in any country, however.