The Gods Forbid by Rory L. Scott

[MAJOR TEMPT THE GODS SPOILERS]
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The Gods Forbid is a villain redemption, enemy’s sister, forbidden romance novel. It is the fourth and final book in the Tempt the Gods series and is not recommended to be read as a standalone, since the reveal of the villain is contained in the third book.

A villain and his redemption. Forbidden.

Sebastian Apollo dared to tempt the gods and lost. Now, he has found himself contained to Prometheus, a famed prison at the end of the world. Except now, Prometheus is run by the little sister of the god he tried to overthrow, and is focused on redemption rather than punishment.
Persy Jupiter has spent the last four years attempting to turn a famed prison into a safe haven for redemption and has succeeded. Until her brother plops the god who tried to overthrow him and almost killed his wife into her protection, asking her to accomplish an impossible task—bring him back as Apollo.
As Persy tries to help Sebastian, Sebastian will learn what happens when you try to tempt the gods and then becomes responsible for fixing his own mistakes. As six months fly by, Persy and Sebastian will resist the pull to each other, knowing that their relationship is the very thing the gods forbid.