What can I tell my sister?

"Career path"? She's 12! Look, just let her dream! You need to apologize to her for trying to crush them, not for being mean. And ask your mom what she meant. Tell her you've apologized, and then tell her that you've been really worried ever since she told you to stop acting as if you can sing. Ask her if you yourself are tone deaf. If it's been bothering you, then talk about it. I'm sure if she didn't mean what she said, she'd say so.