Team Whedon has freely admitted that Spike’s vampire lineage was adjusted mid-season, but the explanation they came up with works just fine for me. There are plenty of people who call their step-parents Mom and Dad, so it seems fitting that Spike would refer to the guy who actually “raised” him (i.e. taught him the vampire rules) as his sire.

A couple more small quibbles — Firefly came before Fillion, Torres, and Baldwin were part of the Buffyverse. Joss cast them as villains in the final seasons as a way to work with them again after their show was so horribly short-lived. And the Dollhouse unaired episode ‘Epitaph One’ isn’t a prequel; it’s set in a post-apocalyptic future. [/nitpicky fangirl]

One addition/correction I wanted to add, pointed out to me by a reader: in Spike’s first episodes, Angel is referred to as Spike’s sire. But really, Spike was sired by Drusilla, who was sired by Angel – making Angel Spike’s grand-sire.

The reason I made this mistake is because I had just seen the episode in Season 2 when Spike arrived in Sunnydale and raided the high school on parent-teacher night: Angel took Xander “fake hostage” and presented him to Spike, who called him his sire. (Xander is confused and makes a big deal, “What’s a sire?”)

When Dru shows up and Buffy gets jealous, Angel explains that he sired Dru. Or as the Wikipedia page puts it, “though Spike often refers to Angelus as his sire metaphorically, Spike explains ‘Drusilla sired me… but you… you made me a monster.’” http://en.wikipedia.org/wiki/Spike_(Buffy_the_Vampire_Slayer)

Maybe Angel siring Spike was always meant “metaphorically,” but in that first Spike ep, Spike seems to use it literally. He also calls Angel “ANgelus,” instead of what they’d later use, “AnGELus.” I think it shows the evolution of the storyline for the season and for the Angel and Spike characters.