ask your brother to change the password, with no reason, it is the easiest solution.
My bro is really sharp. He's gonna wanna know why. I could just lie and say he's changed it to my sis in law and at least that issue'll be put to rest.
Still, my sis in law is gonna be on alert. I think the mystery girl just might be someone in his study group, or maybe she is some flirtatious skank he met platonically who initiated those texts. I dunno...