While I agree that it is about condolences after his brother's passing, I also heard the sadness being felt and thought that should be addressed. I suggest that is the reason for a letter that will give the OP some peace of mind. As others have posted, do what is right for you to feel comfortable with your own perception of the situation. Try to find a way of simply letting it go. Having regret and sadness will keep the relationship prolonged in your thoughts. All that any of us can do, IMHO, is wish others peace and comfort that meets their needs. If your half brother simply tosses your letter aside, so be it. That is his choice but the family and, more importantly, you, will know that you have done all that one can do and then simply let it go.