I was reading a manga late at night and forgot to bookmark the series. It's about a girl who seems to have kidnapped a young girl. The young girl does a charm every night as if both main characters are getting married. They say a vow about staying together forever. The kidnapper is a yandere type character who manipulates and intimidates everyone else who threatens to lifestyle. Meanwhile, the kidnapped child's brother is searching for her.

Yes it is, and it's one I started to read recently and then put down because it seemed it could only end tragically.