Seems the answer is in your post... if your brother is expecting a finders fee for work referred to you by him, then he should be willing to do the same for the work you refer to him... if not, on what basis does he think he can approach it so one-sided as brothers?.... So it seems a conversation with your brother is the next step...
That said, happens all the time in contracting...
Keep in mind though, that's different than him contracting with the GC then subbing it out to you... makes him ultimately responsible for performance in the same way the GC is responsible as his contract would be with the GC and your contract would be with your brother... that entails a mark-up...
Your brother, as a "remodeler" is just higher up on the food-chain in the construction world and generally would act as a GC, hiring subs (i.e. - painters, electricians, plumbers, etc.) and would be used to marking those services up for the responsibility and management... while not unheard of, you generally don't find many painters whose main business is painting GC'ing projects and taking on the responsibilities that come with it... otherwise, they wouldn't be painting, but GC'ing generally being the more profitable of the two options...
Either way, whether your brother is the one who is contracted and then subs out to you or you pay him a finders fee, the money is going to either be tacked on to the quote, or it's going to come from the only other place it can come from if not from the project as an added line item...
... YOUR pocket...