I HIGHLY DOUBT it was because you use the same internet. Jagex does not actively search everyones accounts for ips that match to my knowledge, that would be a rediculus waste of time. They would however, if you were reported for account sharing. What I believe happened, was your brother said something in public or private chat, something like, "BRB getting on my sisters account" or something like that. A random person then reported him probably.