Is it permissible for the paternal aunt to perform the aqiqah for her brother's daughter, who is seven years and three months old?

The 'Aqiqah is a confirmed Sunnah (Prophetic tradition) incumbent upon the guardian of the newborn on the seventh day. If not possible, then on the fourteenth, and if not, then on the twenty-first. If the guardian delays, it is Sunnah to slaughter for the child at any time until they reach puberty. If they reach puberty and no 'Aqiqah has been performed for them, they may perform the 'Aqiqah for themselves. It is permissible to donate the 'Aqiqah for your niece if an 'Aqiqah has not been performed for her, while it is recommended to seek permission from the one upon whom the 'Aqiqah is obligatory, as a way to avoid scholarly disagreement.