Fake / Imposter Romance

FL's sister runs away before her marriage to the prince. FL is her twin, so she is forced to take her place and marry the ML.

FL was a maid, and was ordered to have sex with the prince acting as the princess because the princess was no longer pure, but the FL was. She delays consummating and slowly gets to know the prince.

FL takes the place of her brother at school to save herself and her family (she had previously died at the hands of the prince, and right at death she is sent back in time)

FL dies and wakes up as the princess in a story she's read. Therefore, she knows that she's a fake and gets executed in the story - and that the real princess is actually her maid.

FL is a daughter of a maid but the maid switched the babies. She learned really early on that she was the fake daughter of the duke, and her 'mother' (the duchess) never showed her any affection no matter how much FL tried. FL is the illegitimate daughter of the duke (so the duke had an affair with the maid aka FL's birth mother). The duchess wants her real daughter to take the FLs spot - giving the real daughter all that FL accomplished, leading FL to want to get revenge