Well from what we've seen the mother was pretty much a complete innocent in all this. It was Harry
who appeared to shape the futures of Dexter and Deb the most, either through his law enforcement career or through the "code" that he taught Dexter.
In any case, I can't say I have much interest in exploring Dexter's past anymore anyway. By this point, the story should have been focused much more on whatever future
he might have, and with his big showdown with the police. That's what we've all been looking forward to the last few years.