just found out that my twin brother doesn’t have the same dad as me

i’m very hurt by this for some reason. apparently my brother has known for 15 years that we don’t have the same dad. my mom just told me this last night for some reason, she’s getting older and i think she just telling me shit she feels guilty about now because she’s going to die. my mom literally was just at a family event talking about how my brother looks so much like my grandad, while knowing that he isn’t even related to my grandpa. i’m not necessarily close to my twin brother at all and i remember us having an odd shift in our relationship when we were children, around the age my mom told him that we have different dads. my dad has known this entire time, no one ever thought to tell me