The matter has not yet gone to court. But are things going before the court imminent? If so who will be going to court, you or they? If and when matters actually land in court your questions as well as the answers you get here will be different. My advice would be if they ask you to keep quiet, you keep quiet. Still if they go to court, if you feel so, come here again. Do not open the Pandora's box in your own. Silently and carefully preserve all the evidences you have. Present laws were made at a time when there was no artificial insemination. One cannot determine who is a legitimate child or who is an illegitimate on the basis of those laws. Pandu, Dhritharashtra and Vidura were born through artificial insemination though not using instruments. Questions similar to yours were asked in Simhasana-dvarthrimsathi. Excellant answers were also given. Under ancient Hindu laws legal father and child relationships have preponderance over biological ones.
By the way is he your elder brother or younger brother? Are you married? Have you got your own children