Board Thread:General Discussion/@comment-69.254.216.183-20130417022636/@comment-5917342-20130417023355

In some quest she must be your follower no matter you already have one or not. If you cannot dismiss your current follower, try to do the Dragon Seekers quest for Farkas. He will replace your current follower and leave you when the quest is done, so you'll have no followers after this quest, and you can recruit a new follower.