How should the questioner deal with her sister, who tries to diminish her self-confidence and beauty, despite their mother's attempts to calm the situation, and what is the way to deal with her hurtful personality?

Advise your sister to treat her siblings well and avoid harming them. Be patient with her and do not sever ties with her, for severing kinship is forbidden. Overlook her mistakes. Know that responding to evil with good and using kind words fosters affection and breaks enmity, as Allah Almighty says: "Repel [evil] with that which is better; then behold, he between whom and you was enmity [will become] as though he was a devoted friend." Occupy yourselves with what benefits you and concern yourselves with noble matters.