Before Dawn was added to the cast, I liked the fact that Buffy, Willow, Xander, and even Cordy did not have siblings. It was quite a fascinating coincidence. I'm not too sure about Oz, Wesley, and Giles but they didn't seem to have any too.
Anyway, I was really annoyed with the inclusion of Dawn. She was needed for the story to work but she was just too mean and selfish for someone who is supposed to be an "innocent". If the excuse is she's supposed to appear as a normal teenager, well, that's an insult to teenagers. The altered memories also made me angry because it is mind rape. Angel got a lot of criticism in Ats S5 for this same situation.
The inclusion of Dawn also worked against what is canon in Buffyverse, that is, magic is unstable. I was expecting Dawn to weaken or have a crisis of near death because she is a creature of magic. What is sustaining her? Her creator, the monks, were all dead. Maybe one of you has a theory about why she lasted this long without a crisis to her existence which should have been fragile. I would really be grateful.
And yet her inclusion could have worked well with Adam's comment about magic in the episode Superstar. He said that magic results in chaos, that's why he won't interfere with Jonathan's fantasy. If I see it that way, then Dawn's appearance did start the chaos in the personal lives of the Scoobies all the way to S7.:biggrin: