You have to sort out relationships for your question...otherwise appears like you are intending to circumvent the law.
#1. Ordering a firearm from PSA is a relationship between PSA and the FFL.(not you or your brother)
#2. FFL has a relationship with federal government.
#3. The firearm technically belongs to the FFL until transferred to buyer that has requested the purchase.
#4. FFL has a responsibility and relationship with your brother(or buyer) to do a background check,collect taxes,charge for services, and assure the 4473 is accurate and complete.
So, the responsibility for legal sale goes to FFL if firearm or transaction is ever in question. Usually, FFL keeps 4473 forms in his files that are accessible to BATFE.
FYI: The tools to assemble an AR CORRECTLY cost well over half of what a complete unit can be bought for.
Note: That I have witnessed people that claim an AR could be assembled with screwdriver,allen wrench, and channel lock pliers and some form of hammering device.