Last August my sister died of cancer. It happened in an European country, while I live in the U.S. My sister's husband, son and niece, who live in the same city as my mother, opted not to tell her that her daughter died. They told me that they made this decision, because they were afraid of a strong reaction from my 85 years old mother. At that time they also told me that I shall not tell my mother about my sister's death, which i respected. They also told all neighbors and friends that my mother does not know about the death of her daughter.
My mother is senile and forgetfull, but very fit and energetic, walking long hours every day. Currently she needs help only with her medicines. I am talking to her and the caretaker I hired two years ago on the phone very often. From these conversations I know that my mother asks my brother-in-law regularly about my sister. The answer she gets is that my sister is in a hospital in another city. In a few months I am going to visit my mother. Shall I tell her about my sister's death and if so, how? Your answer will be greatly appreciated.
Talk to your sister's husband and your other relatives first. You need to agree, as it will be very confusing for your mother, if you tell your mother that your sister is dead and the others say that she is alive.