What should I do regarding a sister who is semi-uncovered and wishes to wear the niqab, given my fear that she might wear it for a period and then revert to being worse than she was?

The girl must repent immediately and stop displaying her adornments (tabarruj) and wearing perfume, because covering the face is obligatory according to the most preponderant opinion of scholars. It is permissible to advise her, while observing the Sharia guidelines and ensuring that no fitna (strife/temptation) occurs. It is preferable for a girl or one of her mahrams (unmarriageable kin) to advise her.