What should a 15-year-old girl do who strongly desires to wear the niqab, while her niqabi mother asks her to wait for Allah's decree, and her sister opposes her?

Wearing the niqab is a righteous deed, and initiating righteous deeds is commendable. Therefore, proceed with it after your parents' approval, keeping in mind that many girls later abandon it due to environment or weak faith. Thus, the matter must be taken seriously, and one must accustom oneself to prioritizing what Allah loves over personal desires and the pursuit of comfort.