Why would you not be able to sign up for a Nexon ID if your brother played Maple Story? I see nothing in their Terms of Service against having two accounts on the same IP. I could understand if *YOU* played Maple Story, because then you'd already have a Nexon ID. Don't worry, nobody openly admits to being a Maple Story vet, 'cept me, it seems.
As for Mabinogi as a game... I found it to be quite lacking when shown this game sometime back. You can only create a single character for free. After that, it'll cost you money. You're also very limited on customization without forking out money as well. The gameplay is not that great in comparison to other free MMORPG's.
Good thing Nexon upset me many years ago and prompted me to cancel my old Nexon ID. I was able to do this offer! ((The upset came from when the beta for Maple Story ended and all beta characters deleted.)) For me, it credited very quickly.