My brother (same age as I am) is dating this girl from school who is the most wildly "popular" person in school.

This happens to be the same girl that picks on my sister (also same age as us) ALL the time. I do my best to step in and help her but it never works out.

Nevertheless this girl is definitely cheating on my brother, seen proof myself. Problem is would be that my brother and I don't get along very well...so how do I tell him about this girl?! I highly doubt he'd even believe me but I'm at a loss...this is not good at all!