I use Baka-Updates to search for all associated names. Here, so it won't take you 2 days anymore: https://www.mangaupdates.com/series/rp83zig/adopted-by-a-murderous-duke-family
Then what about this one? (I use that too):
https://myanimelist.net/manga/168703/Adopted_by_a_Murderous_Duke_Family
Oh and you can try this site too (you have to Browse either the Manga section or Anime section 1st to search a title you want): https://kitsu.io/manga/adopted-by-a-murderous-duke-family
"암살자 가문의 수양딸이 되었다"
Take 2 days for me to search it