I'll start by saying that if her moving in was his idea ... well, he wanted that noise and he's got to deal with that on his own.
Unless OP's breh is more than secure financially/they are married/she gave him a child, it makes entirely too much sense for his girl to pay something, especially if she pushed for it or they both made a mutual decision that it would be best. Did she at least OFFER? The only real benefit to paying every single bill is the power and authority that comes with it.
Me personally, that whole cook and clean all the time shyt (or whatever she provides in lieu of not contributing to coverage of expenses) doesn't matter that much. I can feed myself. I can clean up stuff myself. Guess that leaves fukking and if that's all she's bringing to my life, I'm basically dating a live-in prostitute. To that I say
If she wants to cook and clean, that's fine and all, but if this is a matter of keeping score, then I'd consider those field goals compared to the touchdowns I'm scoring by keeping a roof over our head and the lights on, etc.
At the very least, the expectation is to cover utilities and her personal expenses if we're living together. In a perfect world, it'd be half but I'm more than willing to be flexible on that. Just help out SOMEWHERE. I'm of the mindset that you don't live with a spouse unless you are, AT MINIMUM, engaged to be married.