I am assuming that when you say "third person," you see Aunt Jennifer and Uncle as the first two people in the poem. If so, it follows that the third person is the speaker of the poem, who remains unidentified as to gender, but must, by virtue of the fact that he/she addresses the two main characters of Aunt and Uncle, be the nephew/niece of the couple. My own read of this is that the speaker is a female, and not just because the author is a woman, but because of the speaker's voice. The speaker notices the details of the embroidery that Aunt Jennifer is working on and also notices that Aunt Jennifer has trouble with the needle and that her wedding ring seems heavy on her hand. Of course, the point of the poem is not who will inherit the panel, but the author does not provide the reader with enough information to do more than assume and create our own scenarios. Perhaps Aunt Jennifer has children of her own. Perhaps the niece is the only relative she has left to care about her. The poem remains ambiguous to give the reader opportunity to ponder and decide for him/herself.