You can have as many accounts as you want, even as a single person. The only time IP address might matter is if one of the accounts gets banned (pretty hard to do) or scams someone (would have to be done on purpose). Even then, if we can tell that they are different people, they will not be effected.
If you want, we can also whitelist your brother's account with a vouch from you. Just PM one of the staff members that are online (if the request is high priority) or post in the whitelist request thread (if the request is low priority).
+1. Tell him he's welcome here.