I keep asking girls out but they think of me as a brother. How can I stop giving a brotherly vibe?

Take acting classes.

Now, quite often people think I told them to prepare for a career in acting, and I didn't. Your problem is you don't know how to act to get the results you want, and you need to learn to act differently. Nevertheless, John Molloy says in his book "Dress For Success" that vocal training will improve earning power more than any other detail, and acting skill will get you a job offer that you would not have gotten without it. So there are good reasons to develop professional abilities even if you don't intend to make your career in acting. Just for one example, a good salesman is always a superb story teller.

Go to a bookstore and ask for Elizabeth Browning. Choose a book that will fit in your pocket. When you talk to a girl and have some time, tell her you are working on voice lessons and you need feedback. Would she listen to you read and critique your presentation? Then sit her on your left side and read poems to her. Let us know how it works out.