Tell your brother the truth. How do you think your brother would feel that his wife alreday had realationship with you when he finds out letter on. If you truly care about your brother then Tell him every thing. If he still wants to marry her then that is between him and her. Otherwise you may ruin your brother's marriutal life and he may never forgive you. It is not to late, He may be angry with you for a while, but he still is your brother. And your brother still have a chance to choose another wife if he does not wish to marry that girl. Be honest and tell your brother, story Khattam.