My brother cheating?

So my brother is dating this girl she is so so sweet and lovely. She's honestly better than any of his exes and we became very close.

Last night she told me that she found out my brother was flirting with another girl. She was incredibly upset and I suggested breaking up but she says she wants to make it work. My brother has been pretty nonchalant about the whole thing.

He honestly doesn't even treat her as good as he treated his previous exes.

What do I do?

A part of me wished my brother would never do that. It's embarrassing to know your own brother does that.