How should he tell her that he wants to marry her?
Question

                I was recently informed of this site, ma'shallah. What is the correct way to approach a sister whom you intend to marry ? I've known this sister my entire life, in the same regard as my own blood sister and recently it was brought to my attention that she would be a perfect candidate for marriage. I believe she respects me, but I am unsure if she can accept me as her spouse. How can I know ? And what is the correct, halal way to approach ? I have nothing but the best intentions, yet my entire life I always felt I would marry her, yet out of respect I have never approached. My father and aunt (my mother has passed, allah yarham ha) are very fond of her and encourage me to pursue her, since they know her and her family extremly well.
Praise be to Allah.If a man feels that his heart is inclined towards a particular woman, he should follow the way that is permitted in Islam, which is marriage. If a person intends to marry a woman, he has to propose marriage to her through her wali (guardian), who is her father if he is alive, or one of her other close relatives.  It is not permissible for a man to propose marriage to a woman who is not his mahram through ways that are not permissible according to sharee’ah, such as getting to know her or meeting her frequently or getting in touch with her often and speaking on the phone; these are means through which Iblees causes people to sin and which lead to regrettable consequences. It is not permissible for a person to justify his relationship with a girl by saying that she is like his sister and other silly excuses. You have to follow the proper channels as prescribed in Islam to marry this woman. When you propose marriage, it is o.k. if you try to win her over by giving her a gift through her wali. We ask Allaah to give us and you strength and to keep us away from haraam things. And Allaah is the Source of strength. 
See also question no.2572.