((tyranover))
I certainly hear you loud and clear. You sound very educated and very smart. You explained your situation well and you sound very level headed about it.
I have a little story for you. Here is the sad thing that you may have to accept. I was 22 (now I am 57) and my mom still grabbed my hand when we crossed a street. She couldn't help it. I know you are speaking about your siblings here. It is the same concept. They probably won't change their opinions because you will always be the younger sibling BUT that doesn't mean that you are inferior in any way. You are the only one who can know that you are mature. They may get it much later in life. You have to know within yourself that you are not so young and that you have good things to add.
I hope this helps in some small way. Believe in your own abilities apart from their actions. This is when you really mature.