Depending on the method your brother used to send the Gift Card, he can resend it, check status, and also review the email address to make sure it was entered correctly. There is also a link in this support article to Contact Apple directly if necessary. On your end, all you can do is make sure it did not go into your Junk Folder.
Gift apps, media, and more from Apple - Apple Support
I assume the country/region of your brother is the same as you as indicated by the region designated on each of your Apple Accounts. You cannot send a Gift Card from one region to another and if that was the case, the funds would be returned to your brother.