She thinks my brother is cheating on her. She says she snooped his phone and saw some texts from some girl that read, "You're adorable. How soon can we meet?" and "I'm wearing close to nothing. I'm freezing."
Well what did the other messages say? We/you need more context. If they were:
Brother: I just saved a bunch of puppies from a burning building. Do you want to meet up so I can give you one?
Girl: You're adorable. How soon can we meet?
Then no. But if:
Brother: I want to bury my face in your beautiful ass
Girl: You're adorable. How soon can we meet?
Then yes, he is cheating.
Brother: I'm really cold.
Girl: Me too
Girl: I'm wearing close to nothing. I'm freezing.
Then no. But if:
Brother: What are you wearing, you dirty little fucking skank
Girl: I'm wearing close to nothing. I'm freezing.
Then yes, maybe suspect.
And I wouldn't stick up for my bro if he was cheating. People have a right to know.