Yes. According to The Code of Canon Law, unlike sponsors at Baptism (Canon 872) and Confirmation (Canon 892), the witnesses at marriage assume no responsibility to attest to the faith of those entering marriage or to assist them in living out the obligations of marriage. Their sole function is to witness to the fact that the marriage was legitimately celebrated. The code stipulates no detailed qualifications for those fulfilling this role. To function as witnesses to the marriage the witnesses should possess the use of reason and be capable of understanding the events they are witnessing. Canon 97 sets the use of reason to be at the completion of the seventh year. In this case it is perfectly all right for your 10 year old niece to be your “maid of honor.”