He is afraid that he will offend his friend if he proposes to his sister
Question

                I am studying at University, and coming to the end of my degree next year,    Inshallah! I have know a brother for a few years now, and we have know to       love each for the sake of Allah (SWT). The brother has a sister, and I wish to propose to his sister for marriage, yet what is the Islamic ettiquette for proposing to this brother for his sister's hand in marriage, as he is his sister's walee?? I feel that he may be offended or angry, as she is precious to him?
Praise be to Allah.There is no
      particular etiquette prescribed in sharee’ah for proposing marriage
      apart from the traditions and etiquette that are known in all lands, so
      long as they are in accordance with the sharee’ah.
      
      
      You want to marry your friend’s
      sister, but you do not know how he will react and are afraid that he may
      be angry, but you do not mention any reasons why he may be angry.
      
       Is it
      only because he loves his sister and she is dear to him? This is not an
      obstacle, for the one whose sister or daughter is dear to him will look
      for a good match for her, and this is a part of his sincerity towards her.
 If it is
      a different matter, such as traditions or pragmatic issues, then we cannot
      advise you without knowing what they are.
 You
      could consult with a knowledgeable person from your friend’s country who
      knows you and him, and who could advise you.
 Or you
      could approach your friend in an indirect manner, such as telling him that
      you want to get married and ask him who could offer you advice about women
      or ask him to tell you who knows about the Muslim women. Perhaps then you
      would see something of how he feels about this matter. Or you could tell
      him that you love him for the sake of Allaah, and explain your wishes by
      saying that this love could be strengthened and the tie made more lasting
      if you were to become brothers-in-law. So you could say, for example, “I
      wish I had a sister whom you could marry, or you had a (female) relative
      whom I could marry, so that we could always keep in touch.” Then wait
      and see his reaction; then you will know whether to take the matter
      further or not. If you are afraid of his reaction, you could ask someone
      else to bring up this topic, so as to spare yourself any embarrassment.
      And remember that du’aa’ is one of the most important means of
      achieving desired results. May Allaah bless our Prophet Muhammad.