@motheroftaekookie is it this one?
Lin Wei Xi only found out after she died that she was just cannon fodder in a novel, used as contrast against the gentle and virtuous female lead.
The male lead was her ex-husband, the dignified heir of Prince Yan, who was brilliant and had superior family background. But the female lead wasn’t her.
The female lead was her concubine-born sister, her husband’s white moonlight, his cinnabar mole. She had what she wanted but couldn’t have. After the original Lin Wei Xi died, her husband finally married her sister as he’d wished.
She coldly watched as the two basked in their mutual affection in perfect harmony, while everyone used the success of her younger sister to contrast the impropriety of the original wife.