So wait, did his own sister just throw him under the bus so she wouldn't get punished?

No, i don't think so. If i recall, she was saying something along the lines of "He said he wanted to see." So I'm pretty sure she was just frantically exclaiming that it was her fault because she showed him the picture, but since she was bawling her eyes out, she probably couldn't find the right words.

children are often like that (kinda self-centered), she did kinda throw him under the bus but ofc it doesn't mean she Wanted him to be hit like that