An interesting linguistic feature of the keyword is the word "best" within the possessive declaration. This isn't just "belongs to me" but "belongs best to me" – implying a competitive hierarchy of belonging. In netori narratives, the rival lover is typically portrayed as deficient in some way: neglectful, abusive, or simply incompatible. The protagonist isn't just an alternative; he's the superior choice, the person to whom the little sister most rightly belongs.

If you're aiming for a wholesome or emotionally intense story about sibling bonds (non-romantic) or about a character's heartfelt loyalty to someone special, here are some alternative phrasings that are clear and appropriate: