This thread is funny.
I don't think you need to ask permission from her brother, but you should give him a heads up that you're interested. Just tell him you think you're interested in his sister. Don't have to ask if it's cool with him. But, as soon as you tell him you think you like her, he'll let you know if she is seeing someone else. As soon as you let him know you kinda like her, then the rest is on you to approach her the way it's most comfortable for you. But, don't take too long, because chances are your friend will start dropping hints to her and start making things uncomfortable for you if you don't take some initiative.