My Sister Is a Former Genius Witch

In a world where there is a small number of witches but their magical powers are not so great, my little sister is a one-in-a-million exception who can utilize all kinds of spells! But because of it, my sister now becomes a exploitative (in terms of using magic) brat who always teases me for not having magic capabilities... However, what would happen when she actually runs out of MP (magical power)?