Hi
I’m looking for an webtoon or manwha where the female lead is the older half sister to the king. Her mother had been sentenced to death a year or two before due to an accusation of witch craft and not the FL is cursed and must have sex ti relieve the pain associated with the curse. She has plans to join a convent to get away from her step mother.

Sounds like The Secret Bedroom of a Dejected Royal Daughter

https://www.mangaupdates.com/series/hzdmyn4/the-secret-bedroom-of-a-dejected-royal-daughter