I am a USA citizen but live and work in a foreign country, should I enter a USA mailing address (my brothers address ) or my foreign living address in my tax return.

You should use the address where you want the IRS to send you official mail, and your refund check (if you are due a refund and there is problem with the bank deposit.)

Your tax return address does not have to match your residence.

VERY IMPORTANT- Some states are aggressively looking for additional revenue. While you are given a choice in TurboTax to supply a mailing address and TurboTax says it doesn't have to be your residence... some states are now going over federal returns for people with mailing addresses in their state. With no other supporting data they are then sending out letters saying "we think you are a resident so refile your taxes and pay us $X plus interest and fees." 

If you use your brother's address that state may now start sending out demand letters.