you are her child.

Rosalie saved her, adopted, and will continue to care for her until the last days of her long life as a vampire. Isabel became her child when she was just one year old, when she saved her from a neglectful mother and an abusive father; of course, she put an end to them in her own little way.

the little girl was a human, yes, but that didn't matter when Rosalie took her away from that horrible family. she could turn Isabel into a vampire when she was the right age, of course, if she wanted too; although Rosalie had no plans to strip her daughter of her humanity, as she knew how... terrible, the life of a vampire was.

and all would remain well if they continued to hide Isabel from the Vulturi clan. despite the agreement they had after Renesmee was born and Belle turned into a vampire; Rosalie wouldn't even like to imagine what could happen to Isabel if they found out about her existence. so she will continue like this, being taught at home, hidden from anyone who would threaten her much-loved daughter, being protected by the rest of the Cullen family.