If you can see the car in your Gift Inbox, but you can't receive it, have your brother send you another item like a paint or tuning part, anything, and then when you get it, click on "Receive All" and it should come through.
Sometimes gifts get "stuck" and the above usually releases it.
If you are saying you or he can't send something, then that might be due to the $1M limit, but you would get a message telling you that you can't gift it for any item that is untradeable.
