What are three good topics to research about Sister of My Heart?

I am a little unclear as to exactly what you might mean by "topics to research."  If you mean themes from the story, I can help with that.  It is possible that you mean research topics in the real world that the book topically brings up.  I will offer a suggestion in that area too.  

For the real world research topic, I recommend researching Calcutta life itself.  The story is about two girls growing...

I am a little unclear as to exactly what you might mean by "topics to research."  If you mean themes from the story, I can help with that.  It is possible that you mean research topics in the real world that the book topically brings up.  I will offer a suggestion in that area too.  

For the real world research topic, I recommend researching Calcutta life itself.  The story is about two girls growing up in Calcutta.  There is plenty that you could research regarding the history and cultural changes that have affected that area over the years.  You would want to focus specifically on matching up the time period with the setting of the book.  

Regarding themes of the book, you could analyze the story as the coming of age story of two young girls.  Another theme would be the social caste system that is presented within the book.  That would tie in with your cultural research of Calcutta itself.  A final, third theme that I recommend is to focus on the power of family as presented in the book.