This is a question that readers have seriously been asking since the book first came out. Some readers have even been a little ticked off by the title because they were expecting this story to be about a nun. If you're one of those readers, we hope you were pleasantly surprised instead of disappointed.
As far as sisters in the book go, Carrie has a sister (who appears in, like, a few measly chapters), which of course also means that Carrie is a sister herself. Also, we learn that her nickname back home was Sister Carrie. But what does that have to do with anything?
As Lee Clark Mitchell notes, the title may have to do with a little tidbit from Dreiser's biography: he's said to have based Carrie on his own sister Emma (who really was kidnapped by her lover who had stolen a lot of money). So it may just be something of a shout-out to his sis.
Criticisms that the title of the novel is pretty arbitrary are certainly valid. On the other hand, when you think about it, the title may be a nod to one of the central concerns of the book: relationships. Specifically, the title defines Carrie by her relationship to someone else. As we quickly find out, Carrie struggles to maintain relationships throughout the novel and ends up sad and alone, despite becoming a wealthy actress. So maybe the title is priming us to think about Carrie's relationships and her loss of them.
One more thing about nuns. They aren't exactly known for their materialism and love of the spotlight, right? So maybe, by setting up the potential for readers to anticipate a book about life in the convent based on the title, Dreiser intended to further highlight the hollowness of Carrie's pursuit of wealth and fame.