If you're looking for manga similar to Omae wo Onii-chan ni Shite Yarou ka!? (Light Novel), you might like these titles.
Kurata Yuki is staying alone at home while his parents are off on their vacation. His childhood friends Nekoyanagi Mei and Inui Sayo are helping him with the household stuff in the mean time. Mei is outspoken and active while Sayo is calm and knowledgeable. They have known each other for their entire life and they treat each other like they were real siblings. Little do any of them know, they’ll be involved in a love triangle a year from now.
In a dramatic reversal of fate, the commoner Ludwik ascended the throne and became king. Once crowned, he was immediately told that the most important work of the king is “to welcome the princess from the highest lineage on the continent as your bride”! But then, the beautiful bride who appeared muttered, “gross…” upon seeing his face. Just like that, the new queen began to wage a cold war against Ludwik. But the truth is, there lies a secret within the queen’s heart, and when Ludwik finds out about it, he ends up in a certain contract with her… The story of fantasy, harem (planned), and comedy begins here!!
A high school senior named Yotaro thought he was an only child — until he met his real mother and discovered that she has 19 daughters, from 0 to 18 years old. He ends up living with his "true family."