Do Jehovah's Witnesses recognize marriages performed by non-denominational ministers? I am seeking my Ordained Minister License and was asked to perform the wedding ceremony of my brother and his future bride, who are studying to become Jehovah's Witnesses.  Do JWs require a particular Ordained Minister License to perform marriages in their denomination?
As your brother and sister-in-law are not yet baptized members of Jehovah's Witnesses it would not matter who performed their legal marriage ceremony. In fact a baptized member of Jehovah's Witnesses can be married by a Justice of the Peace if they so desire. A member of the congregation could even be married by one of the couple's fathers or good friends if that person meets the qualifications set forth by that particular state's laws for officiating a wedding.

Where things would become more complicated would be in two scenarios:

1). If the couple were to be married by you, in a church, as part of a religious ceremony this would be frowned upon by the individuals studying with them and by the religion in general. Because Jehovah's Witnesses consider all other religious to be false, just attending religious ceremonies at a non-JW church would generally be considered taboo. (See "Question from Readers" Watchtower 2002 5/15.) So, your brother and sister-in-law being married as part of a religious ceremony would definitely run them afoul of the congregants with which they study.

Another point that may illustrate the JW stance on matters of interfaith, is that as an individual grows closer to baptism, if they are a former member of another religion, they are urged to send a letter of resignation to that church so that they are in no way connected with what is considered "false religion." (See "Keep Clear From False Religion" Watchtower 2006 3/15)  

2). The other scenario, though less likely, is if you would want to perform the ceremony at one of Jehovah's Witnesses places of worship (Kingdom Hall). In that case it would not be possible as generally only Jehovah's Witnesses Elder's are allowed to perform religious services at a Kingdom Hall. Of course being that your loved ones are not yet baptized they would not be able to marry at a Kingdom Hall in the first place. Only baptized members of the religion, who are in good standing, would be able to avail themselves of the Kingdom Hall facilities.

So to summarize: If you are chiefly officiating the legal proceeding of the wedding (which may include all of the traditional activities associated with a wedding such as reciting of vows and exchanging of rings) then there should be no issues. However if the ceremony also has heavy religious overtones or is held at a church, if your brother and sister-in-law are as far along it would appear, this would definitely create friction for them. Hope this info is helpful!