We may have seen the blonde before afterall. This is a picture from Kori's birthday party in July, 2005. If it's the same girl she might be a relative, family friend and/or nanny for Shar's kids. It would explain why her arm is around Cassie in the group photo at Thanksgiving, and not Kevin. UPDATE: anonymous comment - "She's a girlfriend of a relative. Don't make anything out of it." UPDATE 2: anonymous comment - "those two girls aren't even the same person! the girl in the black is a family friend ,"kfed" new girl is shars brothers fiance!"
Source: ubritney.com, roxi (thanks for finding that!)