That's rather harsh of your sister. o__O
The only thing jumping out at me is is forearm. It's too long, and kind of emerging from the side of the mermaids tail. ^^
It's pretty startling on it's own, but if you want to make it more -so you can make the mermaid look limp. Like, tilt the head so it appears to be lolling back, as though she just doesn't have the energy, or is even dead.
It looks really cool as it is, though. It's a really nifty idea, and the boy has such a lost, "What have I/they done?" expression on his face. heart