There was a huge smack-you-in-the-face foreshadowing with Deb catching Dexter one day.
It might as well have been in bold letters with ringing alarms when Dexter said: "Maybe his sister didn't know," and Debra responded with something to the affect of: "It's always someone you least expect."
I can't remember the exact lines, but the writers were just toying with us there.

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