In my relationship, we'd been friends for years before we started dating. But anyone I saw as an older sibling I would never consider as anything else. Friends can move on, but someone who helped raise me, not so much. Xander was kinda her father figure, as was Giles, and I really am not comfortable with a Dander or a Diles relationship. Dawn and Giles is way too gross to think about.
What comes to mind is the episode where Xander sees Dawn at the Bronze in s7 and starts with his lame, "Daddy like," term, then quickly changes his tune.
Anya always bothered me, but I think the love between she and Xander was incredibly real. Right up until the end. I think, had she lived, they would have been together. Poor Anya. A thousand years as a demon just to be killed by one when she's in human form.