Marrying one's foster brother

Q: I really love my cousin and he love me too. We did everything together and I regret that but it’s happened now. He told his mother that he wants to get married to me. But his 2nd mother told him that she gave me milk and to him also. My mother never told me that even my father don’t know about this. Now we really love each other and we can’t leave each other. What to do?