>>2 This.
>>1 How would a one year age difference make the couple "not very serious" anyway? o-o It's only one mother fuckin' year.
But really, just don't get involved until it's honestly necessary. It's not your business until someone makes a move. Don't defend your brother; don't help your friend. Let the girl take care of herself. If she does cheat and you know it, though, you should go ahead and tell your brother.
Boom, problem solved.