This is what I would say to this young person, whose heart is clearly in the right place:
Speak to your rov or a close family member or that friend you mentioned and let them be the “face” of the shidduch proposal. If it goes well, you can step forward, and if not, you haven’t lost anything. Are you certain your brother would be resistant to hearing of a shidduch from you (and what about your parents, why not get THEM involved)? Are you afraid they won’t give any credence to you because you are so young?
I do want to give one piece of unsolicited advice – just because YOU think the girl is “perfect” for him, does not mean at your age that you necessarily have a sufficient understanding of what “perfect for him,” means. I am NOT saying you don’t know what you are talking about, either. You sound like a wonderful and caring sibling, and very mature in the way in which you present your position. But that being said, 14 year olds and 20+ year olds, have a very different view of life, so what you may think is important for your bro, may not be shayach at all, from his perspective. In any case, hatzlacha rabba, and yasher koach on having your brother’s back. You sound like a really special person.