Kakkou no Iinazuke, Cuckoo’s Fiancee in English, a rom-com manga series written and illustrated by Yoshikawa Miki, is the first series in Kodansha’s history so popular that it sold out of store and had to be reprinted for 4 consecutive weeks!
Myanimelist describes the series as follows:
The story revolves around 2 teenagers who got switched soon after their birth. Hoping to keep both of their biological and adopted children, their parents decided to engage the boy and the girl.
Source: Manga Mogura twitter