How to prove to somebody I didn't do it?

my family is accusing me of doing something which I didn't do, they are saying i went into my cousin cell phone account and blocked her texting, I didn't do it they think its fishy just because i told her that she was eligible for a upgrade and I wanted to her to get a new phone. I didn't do it but I have no actual way to prove i didnt they wont look at my internet history. I just dont know what to do. PLEASE HELP!