Glass no Shiro

The story is set in England. Malissa and Isadora are sisters in a poor family. One day, Isadora gets to know Malissa is a daughter of a noble family, who are searching for the missing daughter. Isadora lies she is the missing daughter to get a castle and all the asset of the family instead of Malissa. After getting into the family, Isadora plans to get married to a crown prince in a foreign country. On the other hand, some people doubt Isadora is the missing daughter. Malissa ends up getting to know she is the real daughter. Note: Won the 1970 Shogakukan Manga Award.