It may be worth looking into to see if there's anything your half-brother can do to re-claim his citizenship. How did he acquire U.S. citizenship? As stated above, nephews do not qualify as immediate relative for U.S. citizenship purposes. There still may be other options for him, such as a nonimmigrant visa. If he (or his wife) worked for a company in the past three years, and that company were willing to send him to the U.S. to work for one of their affiliates (or to open a new company in the U.S.), he may qualify. Also, if he or his wife is British (or from another qualifying country), they could also consider opening up a company in the U.S. and receive a visa based on a substantial investment of ongoing trade with the U.S. and qualifying country.
Please contact a reputable law firm for assistance (note: that most out-of-state attorneys can legally help you because we are permitted to practice federal immigration law outside of our state of jurisdiction).