A few years before my mother passed away, she confided in me and told me a secret that she had been keeping for over 45 years. She said that I was the only person that she felt she could trust and she needed to tell me. She told me that my oldest sister is not my father’s child. She said that she was engaged to be married to my sister’s birth father, but shortly before their wedding, he was murdered. Shortly after, she met my father who immediately loved her and wanted to take care of her and the baby. She was grieving the loss of her fiancée and didn’t love my father but was heavily pressured by her parents to marry before she began to show her pregnancy. This took place in Mexico during the 1940’s. They quickly married and my father brought my mother to live in New York City. My father made her promise that she would never tell my sister that he wasn’t her birth father. She asked me to keep her secret and I did. At the time she told me, she lived with me and I was taking care of her because she had a diagnosis of Alzheimer’s disease. The disease progressed quickly and at some point, she mistakenly told one of my niece’s the secret. Let’s call my niece “Karen”.  Shortly after, Karen told me that my mother told her the secret and I asked her to never repeat it because my mother wasn’t in her right mind anymore. Years later, my sister, my sister’s husband, my brother, and I sat at our mother’s bedside until she took her last breath. She didn’t have a peaceful death. In fact, she suffered immensely and we all have PTSD from watching her die. The morning after our mother’s death, we sat in silence at a restaurant, attempting to choke down breakfast when my brother-in-law received a text from Karen. He silently read the text, threw down his phone, got up from the table and walked away. My sister then read the text. Then to my horror, she read it out loud to my brother and I. In the text, Karen said that I had been keeping a secret from the family and then proceeded to say that our sister was actually our half-sister. My poor sister is sobbing and asks me if it’s true, I was horrified and all I could think of was the tears running down my mother’s face when she told me her secret. It was very hard for me to be dishonest but I denied it. I PROMISED my mother to never repeat what she told me. I was very close with Karen’s sister and I called her shortly after to let her know that my mother had passed. I told her that my siblings and I were in agreement to let the rest of the family know the following day and asked her to not say anything to anyone. Instead, she told her sister, “Karen” who immediately posted it on Facebook. The following morning, Karen contacted our father’s attorney for a copy of the will, when the attorney refused to give it to her, she then contacted the funeral home and said she wanted my mother’s body shipped to where most of us live (my parents were in a high-end Alzheimer’s/Retirement home in another state), she identified herself as my mother’s “daughter” and demanded to know why she was being cremated and instead insisted that she be buried. She told them that she wanted my mother's body sent to where most of us live. I’ll add that my mother raised Karen and her sister. They had very little to do with her after her Alzheimer’s progressed, and never even called the nursing home one single time over the years to check up on her. My father left me in charge of everything after his death and upon my mother’s death. EVERYONE else was explicitly excluded from anything to do with my mother's care or her estate. Karen caused so much trouble and raised so many questions at the funeral home, that the funeral home put a stop to the cremation and instead, my mother’s body was placed in a freezer until I was able to give them proof that Karen wasn’t my mother’s daughter. I had to contact some of our estranged siblings to sign and notarize an affidavit stating that Karen was in fact not my mother’s daughter and had zero authority whatsoever. We had no idea where two of our siblings were living so it took days to track them down. Karen’s sister told me that Karen was so angry and bitter towards me (I stopped helping her financially and with babysitting her 6 kids from 3 different fathers) that she vowed to turn the family against me by revealing that I knew about my sister. It’s important to mention that my half-sister is 100% Mexican as our mother was Mexican and my sister’s birth father was also Mexican. My and my other sibbing’s father was Jewish. A few years after, my sister’s adult children did a DNA test for my sister which showed that we have different fathers. Apparently, they sided all with Karen and now none of them speak to me. Also now, my sister has Alzheimer’s now. Her husband says that their kids don’t even visit them and that I am the only one who visits her. She repeatedly asks about her birth father and has cried many times while talking about the fact that she didn’t get to meet her father who died before she was even born. She speaks of it every single time we’ve visited or spoken over the phone over the last couple of years. Even her husband is mad at me and not so long ago told me that I should have told her the truth years ago. Her type of Alzheimer’s has progressed very quickly. It breaks my heart that she even has to know about her birth father at this stage in her life. In her disease, she is grieving the loss of her birth father over and over and over. I deeply resent her grown kids that felt the need to prove to her that" Karen" was right. One by one, they eventually cut me out of their lives. My sister doesn’t even remember some of her own kids in conversation when I ask how they are all doing. It must weigh so heavily on her because somehow, she hasn’t forgotten about her DNA test. I am sad for my sister that she has this loss to grieve at this stage in her life. I made a promise to my mother. I have zero regrets that I kept her secret and I honestly don’t care what my my nephew's think about me. I never had children and my sister's sons were very dear to my mother and to me. We were a very big part of their lives when they were growing up. The fact that they cut me out of their lives over this says it all to me. My siblings all support me and that’s what matters to me. AITAH for not telling my mother's secret? What would you have done?