Help! My 8 year old brother texted "I hate you" to my girlfriend. Or did he?

The text came from his iPod while my parents were out of town (I was out for a few hours) and he was being babysat by the teenage neighbor, who also had her 2 younger siblings with her. I know the babysitter's younger brother can be a pain, and I think he grabbed my brother's cell phone and typed "I hate you" followed by a weird conversation, saying "just joking LOL" and more. I showed the conversation to my little brother and he said he didn't text that. Somehow I believe him, I know that's not his type of humor; he doesn't even care about texting. So I'm 50% in doubt. My girlfriend can handle that, she knows it's a joke. Should I tell the parents of the neighbors? I don't have concrete proof and I don't want to seem unjustified or accusational without proof. I know the babysitter is very responsible, but her brother can be a stinker. so. What would you do?