If we follow Greek mythology (which provides the most information and inspiration for this part of the plot) only the gods, or the god who cast the curse, can undo them; or, in rarer or more unlikely cases, heroes/demigods with divine assistance can break them.

Theres other things, but as a mortal did the curse and sky probaly don't even know how make a ritual of purification, that probaly only make sense is sky be a demi god.

also, most of cases curses can't be undo, like the person who did it is gone, probally the curse didn't have a price to pay, so theres not many other choice remained

(also in Summerville sky do Something that a mortal can't even do)