But that wasn't the theme of s2. The theme of s1 was family. The theme of s2 was Angel and his past. The two significant figures in Angel's past are Darla and Drusilla because they represent how he became a monster and what he did as a monster. Spike is not relevant to that story. He didn't sire Spike. He didn't torture him as a human, drive him crazy and then damn him when he was a monster.
Angel and Spike have a history (but really in the grand scheme of things they just hung out together for 18 years and from flashbacks it seems many of those years Angelus was pretty annoyed with Spike's antics). But that history is not really deeply ingrained in Angel. The fascinating point of s2 is Angel's desire to save Darla when she was the one who damned him, that was to him tied to whether he was capable of being saved himself. And the irony is that his biggest regret, the worst thing he ever did as a monster (Drusilla), is the very reason he fails. Again - Spike doesn't factor into the story. Spike just isn't really all that relevant or important when it comes to Angel's past. In fact, I would argue that Spike being any part of Dru siring Darla would take away from the perfect poetry of the story. Because Dru siring Darla alone circles back around to Angel's past transgression being the reason Darla could not be saved. If Spike had helped or had anything to do with that, it eats away at the perfect symmetry of it all.