Praise be to Allaah.
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.