Some Вопрос about Dawn, Buffy younger sis..

Эй, guys! I'm just on 5th season episode 4 and since Dawn appeared on the Показать I'm asking myself question, where she was all these years before ?! First I thought its Buffy's half-sister from Buffy's dad another relationship but no, coz Joyce is the mother of them both..so its kinda strange coz even after marriage divorce Dawn should stay with Buff and Joyce and even if not there should be сказал(-а) something about her since the beginning of the show..and in season 5 she just appears suddenly like out of nowhere...and if her story is told in Далее episodes of season 5 I ask for NO SPOLIERS ;)