Here, you would need to add the preposition "in."
The sentence should read: My elder sister is really interested in the problems of the environment.
Other prepositions would not make sense and could confuse the reader as to the intended meaning.
Citing passages from a text is one way to provide elaboration in an essay.
The reason why this would be your answer is because when you cite your source, it allows to person to go to the source to see more information about the quote you got, and not leave them empty handed. If you didn't cite you source, it would be considered plagiarism, and wouldn't allow the reader to get to know more about the quote and what happens.