My brother wants me to keep his addiction secret from our mother

Dear Delyara, My younger brother is addicted to cocaine. He finally told me after losing his job and owing money. He has started treatment and made me promise not to tell Mum because she has heart problems and he says the shock could hurt her. I agreed. Now every Sunday, Mum asks why he has lost weight, why he no longer visits, why he needs money. I lie. I say work is busy. I say he has the flu. Yesterday, she looked directly at me and said, “There’s something wrong with your brother, isn’t there?” I said no. She nodded, but I could tell she knew I was lying. My brother says this is his information to share when he is ready. Mum calls me the responsible child and asks me to look after him. I already am, but in a way she is forbidden to understand. I am carrying his secret because I love him and lying to our mother because I love her. Both loves are starting to feel dishonest.