Congratulations on the purchase. I'm also working on the same strategy, looking for a house now and going to rent by room to cover my living expense or get paid living there, then save down payment for more properties. I think if the younger brother at age of 23 is understandable enough should get what he pays for. This could be a good education for him also that no such a thing as free lunch. You get what you pay for sometimes less. Or he can help you out around the house, such as being the "property manager" for it, finding and screening tenants for you, taking care of repairs and maintenance, cutting grass, paying some bills, etc to fill the gap. Set the rules up front otherwise he's out like everybody else. Even though this is your primary residence, treat it like a business. It could be a good learning experience how to be a landlord. One of the things to learn is how to say NO. I hope it makes sense and you find helpful. Let us know.
Lin