I remember the first chapter is it’s the female lead's wedding day. Her cousin poisons her and she goes to the altar and dies.
She’s then returned back in time though I don’t remember if it was just back to the wedding day or before that. Her parents were killed years ago and she was living with her cousin and aunt and uncle(?) and her cousin made everyone outside think she was stupid and didn’t know anything. They also wanted her to not know she had great power.