they have never liked you, then why take their criticism to heart? If it wasn't your hair, it would be something else they would find fault with. You won't be able to please them so, for your father's sake, just let the comments go over your head. It seems that those you mean the most to you - your brothers and father - love you for who you are, so ignore the others.
I'm sorry for the loss of your mother but perhaps she has a sister or cousin or close friend still living who could be a mentor for you? If not, by keeping your hair long you are keeping a part of her alive too - for yourself and for others who knew you. Perhaps that will strengthen you in your resolve to have long hair.
And no, you're not silly for wanting longer hair at your age. I know of some women who don't find the courage to grow their hair long until they get to your age. The rules about hair and age have more to do with making money than anything else. Just think, if every woman over 30 suddenly stopped going to the salon every 6-8 weeks where that industry would be.