We are the only Orthodox family in our town and do not live near any Orthodox congregation for us to be able to do Hataras Nedarim. Can we do it amongst our family members, such as to have me the father and another two of my sons do the Hataras Nedarim on behalf of the other relatives [we have a total of six adult sons and sons in-laws at home for the holidays, and other male relatives are joining us later to make a Minyan for Rosh Hashana], and then the other relatives will serve as the tribunal to annul our vows. Basically, is it valid to perform Hataras Nedarim if the judges who annul the vow are relatives of each other or of the person asking for the annulment?
Yes. It is valid for relatives to perform Hataras Nedarim on behalf of each other, whether the three judges are related to each other or are related to the person asking for the annulment, or are related to both. Thus, you and two of your sons may annul the vow for your other son, and so on and so forth until everyone has done Hataras Nedarim.
Although relatives are prohibited from serving as judges, if they are relatives of each other or of one of the parties, nonetheless, releasing one from a vow does not carry the same status of law as a judge who sits in judgment, and hence many of the regulations and limitations applicable to judges in a court of law do not applied by the process of Hataras Nedarim.
Sources: See Michaber Yoreh Deah 228:3;Abayey in Nedarim 77a; Shach 228:7; Taz 228:5; Piskeiy Teshuvos 581:16