Is it permissible to boycott one's sister as a form of discipline, knowing that it affects her, and is this boycott considered the forbidden quarreling?

It is impermissible to forsake a sister if the reason for the abandonment has ceased, because the default rule is the impermissibility of forsaking her unless there is a legitimate Shari justification. If the reason persists, then abandonment is permissible, while considering what is most beneficial. If abandonment is the most beneficial course, then there is no objection; otherwise, reconciliation is preferable. It is feared that boycotting for an unlawful reason may prevent forgiveness, as stated in the Hadith: "Except for two who are in dispute—meaning, hostile to each other—or one who severs ties of kinship, meaning relatives, by way of harm or abandonment, each of them is delayed until he returns and desists." It might also be better to speak with her due to her influence and to gain the mother's satisfaction and happiness.