Poe chose to characterize Roderick and Madeline as twins, not simply as brother and sister. Why do you think he made this choice?

Because they are both wacko and even look alike. I don't have the text in front of me but the narrator talks about how similar they look and that both had similar personality dispositions.

Certainly there is something crazy going on. This story has all kinds of speculation to it. Did Roderick kill his sister? Are they even the same person??