but they both are really judgey and sibling-y abotu each other's kills.

Ha, this works as well! I was originally thinking Deb couldn't be a serial killer because you need to preserve the 'oh, no, what if Deb finds out?' aspect, but I do enjoy the idea of Dexter's motivation shifting to 'I can't let her find out; we have totally different approaches and she'd really judge my murders.'