Me and my cousin both have accounts on this site and clicked multi-account on my profile (created after my cousins) but now my brother wants to crate an account. I was wondering could he create an account?

Yes, just make sure to submit a ticket to register as multis.

How do I do that?

Syn means submit a support ticket per the instructions on the Character Creaction Rulespage: http://www.swcombine.com/rules/?Character_Creation#Multi_Accounts

If you click the Wrench on the sidepanel on the right, at the bottom is the Support section. You'll find "Contact Support" there. http://support.swcombine.com/

Its at the bottom of every side panel.

Submitted a ticket so does he create his account and list both Handles?

He can create his account anytime, just include both handles in a ticket and you're good.

Does he still list handles when it mentions multiple accounts in household

Yes otherwise the ASims don't know which accounts are supposed to be ok.