19 year old here who hasn't come out, so I can't say a whole lot from personal experience. However, I can near confidently say that time is the best way to change. Be open with your brother and grandparents, maybe not specifically about yourself but the state of the world we live in. Things are always changing. Eventually, they will likely come around. It might take until your 19 or older, so have patience.
My LGBT-phobic mother has had such thoughts for my entire life. However, just recently for the first time it sounded like she was vaguely open to the idea of nonbinary identifiying people - and this is because of a friend of mine who she has met twice. So, time and small exposure is my answer.