The Duke’s Imposter Sister

Following an uprising, the nine-year-old princess of Grandia, Gris Benedict, had gone missing. In fact, she ended up working as a maid in a brothel, really no better than a slave. She spent eleven years in that hellhole but was fortunate enough to escape her prison. Apparently, she looked similar to the youngest daughter of the House of Byrenhag who had also gone missing in the woods at a young age. Her "uncle" promised to help her escape the brothel if only she pretends to be the missing young lady, and Gris accepted living a lie in order to survive. One day, she is gifted a small, cute animal by her fake brother, the Lord of Byrenhag. She figured it would be her first and last present from him, but then he kept bringing her bigger and bigger gifts… If ever the time came that he try to gift himself, Gris foresaw that their relationship would surely end in a catastrophe.

Note: The webtoon was released as R15, it's not R19 like the novel