Getting into USMA is hard enough with parents' support, even more so without it. But by coming to ask, you've shown a clear sense of caring and initiative, both of which are fundamental qualities for future officers in the US Army. I would try and identify why they do not want you to join. For example, are they afraid of you being injured/killed, are they anti-military, do they fear that they will not see you enough at USMA, etc. I know my mother was/is afraid of me being injured/killed when I go on to serve after graduating USMA, but she realized (after many many talks) that this is my life and what I felt as my responsibility to serve this great nation. I would recommend trying to find out why, which will enable you to not only understand their perspective better, but also to explain why attending USMA and becoming an officer is so important to you. Best of luck to you. If you ever have any questions, do not hesitant from "private messaging" me, I would be more than happy to help in any way I can.