It could be  crime if they knew it was not her cell phone (i.e. that she had borrowed it) and took it anyway; in that case, they took a phone which they would have known they had no right to. On the other hand, if they thought it was her phone and that they had the right to it (e.g. they pay for a phone for her, and even thought she uses it and thinks of it as her phone, it really belongs to her family), there would be no crime. That said, you could still sue them to get it back or to get its value if they do not return it to you--even though there would be no crime if there was no criminal intent, they still cannot keep property belonging to another.