A brother's best friend is the platonic love of many. Rarely this love is reciprocated and when it happens it is a chaos of feelings. Going out with him or not is a very difficult decision, because your brother's feelings are in between. That's why I will leave you the pros and cons of dating that forbidden love.
Sneaking is fun
Everything is more fun if prohibited. If you decide to go out with him, at first everything will be a secret and you will have to hide it from your family. He will be your love secret and you will spend great days with him.
Your family already knows him
You won't have to worry about whether your jealous brother or your family likes the boy you go out with because they already adore him! That in case you want to make your appointments public.
It could be the love of your life
It is possible that fate is finally giving them the opportunity they both sought for years. He is not your best friend, but his relationship is quite similar. They know each other very well and they already have a certain affection.
You know his past
Although it looks like a double-edged sword, it has more advantages than disadvantages, because you know exactly how it is and what to expect from it.
Seeing it with perspective is uncomfortable
If you are an empathetic girl, you will think "If my brother goes out with my best friend, I would be very angry." We must not do what we do not want them to do to us. In the long run it will become uncomfortable for you.
You won't be sure if he really likes you
All your childhood or adolescence, you thought I only saw you as a small and naive girl. What if she still does? He knows about your vulnerability and your head will think you could take advantage of it.
You will get jealous
Your and he's relationship has always been close. Several days a week you see it in your house, even your brother tells you about his life. But if you go out with him you will also find out about the other girls he dates and could break your heart.
You can ruin your brother's friendship
I think you should overcome your brother's feelings first. This means staying with the desire to date the boy. Because if you do you can take it badly, end your great friendship and get quite sad.