Okay, so I know someone who...?

Alright. There's this girl; thirteen. Her name is Ariel. Most teens or children hopeful to be one of the opposite gender like mermaids because there is a tail, so you can't really tell. it's a jealousy thing, but they like mermaids a lot. But from being picked on her entire life about the whole 'little mermaid' joke, she dislikes mermaids. She feels like a boy, dresses like one, talks like one, and does pretty much everything a normal boy would do. Execpt you know, restroom stuff. Her parents are not accepting to LGBT teens or transvesites. Say I'm related to this girl, as a sister or so. How could I help her? She is confused, or so she says, about what to think and how to act. She obviously wants to be a boy, but she gives mixed emotion scences at times. How can I help and get her parents to understand her, and whats going on?