There could be another side to this that your parents may not have considered..that the parents of your niece probably won't appreciate the comparison either and are not happy that it's a big topic of conversation. I say this because my son turns 18 in September and at least two young men will be putting their papers in around about that time. But mine won't. He's autistic and has moderate learning difficulties. He wasn't baptised until he was 11 and he does hold the Aaronic Priesthood. He's done baptisms in the Temple (probably the first time the Temple workers had seen a young man in the baptistry wearing ear defenders!). He blesses the Sacrament, goes home teaching with his Dad, but that will probably be the limit of his service for the foreseeable future. For most of the time I'm grateful for the lovely young man that he is, the blessing he is to our family, but occasionally, just occasionally I do feel sad that he won't be serving a full time proselyting mission. I rejoice in all the young people in our ward receiving their calls and going out to serve, but there can be a tinge of envy. Your parents may not know what to say (my mum is often unsure), but I can understand that it could well be a source of frustration for you.