If the OP's brother had put them in a storage unit to which he alone had the key, they would still be in his possession. But the OP now has possession of them, because his brother "left everything with [him]."
There are two key points here: first, under federal law, the issue is who is in possession of the firearms, not who owns them. The OP is in possession of them. Second, his brother is now a resident of California, and that means that transferring them back to him is an interstate transfer, and must go through an FFL.
Federal law makes no distinction between this situation and buying a gun from a private party: if both parties are residents of the same state, no problem: you can make the sale directly, and no FFL has to be involved. But if you reside in different states, you
must do the transfer through an FFL, regardless of whether the firearms are shipped or one of you travels to the other's state.
Frank's
post #3 spells this out in detail.