That's a really good point. Before slaying Buffy was meant to be a mean girl, and she wouldn't have been happy to babysit a little sister. Dawn and Buffy didn't have a great relationship when we first saw them, and Buffy was more grown-up then, so I bet they could have really disliked each other prior to moving to Sunnydale. Buffy always seemed jealous of how close Dawn was to Joyce, and I bet that put strain on their relationship too.And lets not forgot that we don't know what Dawn life was before they made her . She could have been ignored by Buffy a lot which is one reason she was acting up . I think that Buffy pre -Slayer didn't care for her all since she was stuck up . And Buffy might had started being nicer to her when they moved to Sunnydale & seeing Buffy start to hide things from her again I can really see why she was doing that .