About "più grande": you probably have the answer by now. But for anyone else asking it, i think a good answer might be here (from a native speaker); i had asked a similar question on another thread: https://www.duolingo.com/comment/24472947$comment_id=24474835
So in this case, if it was meant to be comparative i.e. "older sister", it would've been something like "mia sorella più grande di me" (but it should be redundant to say this since we can simply use "sorella maggiore" as i mentioned above) - the omission of the article "di" changes the meaning of "più". Anyone, please feel free to correct me if I'm wrong, thank you!