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.

Happy Sugar Life

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