Complex sentence: Season five introduces a younger "sister" to Buffy, Dawn, who suddenly appears in Buffy's life, and although she is new to the series, to the characters it is as if she has always been there.
Split into simpler sentences: Season Five introduces Buffy's younger "sister", Dawn, who suddenly appears in Buffy's life. Although she is new to the series, to the characters it is as if she has always been there.