Ellone is not Raine's child. Ellone was orphaned, and Raine adopted her. This is proven by the fact that Laguna speaks to a picture of Ellone's parents, saying "Ellone's mom and dad, Ellone has been a very good girl today.", and the fact that Ellone calls Raine by her first name, and not 'mom'. Squall calls her sis because she's very much like a sister to him, that's all. No blood relation.