Big brother and best friend?

Hi everyone. What it is is , yes I may sound nosy but tbh at the moment I don't care I went into my bros facebook as I knew he and my best friend had been contacting & just found out they had been flirting & have not told me nothing about it! Don't know whether to mention it to them & how to throw hints without them knowing I logged into his account? He is too old for her & how they could keep this from me has really annoyed me!