Kaze to Ki no Uta Sanctus: Sei Naru Kana

Both are about a kind of homosexual realtionships, and both are set in somewhere in Europe, in the XIX century, or similar.... In general, the aesthetics is similar. 

Both treats homosexual relationship in a melodramatic tone and has an European aesthetic (setting in Onisama-e is XXth Japan but the girl live in a boarding school.  

This one is highly recommended if you are looking for something like Emma. As Emma is basically focused on social life of Victorian era, how it express the daily life of poors and aristocrats, what they feel, how much they are different and their interactions. And a wonderful & absolute realistic love story. Kaze to ki no uta is also focused on social life of Victorian France, mostly their school life in anime, school system, politics in school, feelings of students, teachers, how daily life goes, breaking of taboos, aristocrats lityle secret..... tho anime is just 1hours.... not much things are showned, manga is full of french life. difference, Emma is a love story of one woman & man but Kaze to ki no uta is love story of two boys. Emma is static, kaze ki is about more dynamic interactions. read more 

The aesthetic is diferent but in both shows the angst and tragedy are the focus. Both have homosexual relationships, on AnK there were more explicit and KtoKnU have more drama. If you are watching for great Boy´s Love Classics, these two are the option. 

While the main theme of these two series is considerably different they both take place in Pre-Industrial France and cover their respective time periods with some historical accuracy. In both there is the theme of social injustice and the scars it leaves on people. There is plenty of drama in these series and the artwork invokes an old school ambience that is very similar. Furthermore, both deal with forbidden loves; Rose of Versailles deals with class differences while Song deals with the growing romantic attachment between two boys. These are bittersweet anime through and through. RoV focuses both on political aspects as well as individual conflicts with plenty of history, and Song deals almost exclusively with the tortuous relationship between the two main characters in a very slow paced and poignant manner.  read more 

Shounen-ai romance with rather old fashion character designs ensures a firm kinship between these two titles. Both deal with coming to terms with same gender affection. Song is considerably darker and extremely heavy on melodrama to the point of being painful to watch, while Lesson XX is more focused on the romantic aspect; but they are both prime examples of old yaoi and probably share the same fan base.