I hate the thought that someone in your house would have done that to her.

If she doesn't want to talk to a prefect or Madam Hootch do you think you could persuade her to talk to Professor Brutka? He's kind, and he might be able to work out who did it. Or if she wants another pet he might be able to charm it in a way that would keep it safe. He put a charm on Sebastian to make it so the cats wouldn't notice he was there and chase him. I don't know if there's a way to keep a pet safe from people.