I would like to ask my brother to officiate my wedding but he is a baptized Jehovah's Witness and my fiance and I are not. According to the beliefs of Jehovah's Witnesses, can he legally officiate our non-denominational ceremony without putting a strain on his faith? If so, how?
Jehovah's Witnesses have not published any article specifically on officiating non-Witness weddings, but there has been an article discussing whether or not Witnesses should attend non-Witness weddings.
The concluding paragraph summarizes the article well:
After carefully weighing all the factors, some Christians may decide that it would not be improper to attend a non-Witness wedding as a quiet observer. But if a Christian reasons that by being present, he might be tempted to compromise godly principles, he may conclude that the risk outweighs the possible benefits. If he decides not to attend the wedding but to go as an invited guest to the festivities held thereafter, he should be determined to “do all things for God’s glory.” (1 Corinthians 10:31) In making such decisions, “each one will carry his own load” of responsibility. (Galatians 6:5) Whatever you decide, therefore, remember that preserving a good conscience before Jehovah God is vital.
So in the context of deciding to officiate the wedding, it would also be a matter of conscience for the Witness. It would depend greatly on the spirituality of those in attendance, and what they may expect to see at the wedding and the following ceremony. If, for instance, people were getting drunk or dancing crudely at the ceremony, it would very likely violate the Witness' conscience. Jehovah's Witnesses also do not participate in interfaith activities.
Legally speaking, one of Jehovah's Witnesses would obviously be subject to the same laws as any other person officiating a wedding.
One caveat: if the Witness happens to be an elder in the congregation, they are specifically instructed to only officiate weddings that are between a man and woman who are both at least unbaptized publishers. - Source: my dad who is an elder.