Sharing same mother but different father

I just found out that I do not share the same father with my elder brother, it is really hard to digest, what should I do?

You have been lied to by people closest to you, and you are certainly entitled to feeling betrayed. Unfortunately, family secrets are more common than you think. Keep in mind that your brother is still your (half) brother, and he is certainly not to blame. As for your mother, she made a mistake by not telling you earlier! Talk with her, if she is willing to talk. Be honest with her about your feelings. If she has been a good mother to you otherwise, try to remember that parents make mistakes too. Very few people lie willingly - they do so under pressure. You may never feel exactly the same about your family again, but don't let this ruin your life.