My condolences on the loss of your brother. This situation is making matters worse for you and stopping you grieving in the way you should be able to. I think simply, if you put yourself in her position, you will know that you would be deeply upset and offended at not being told at the same time as others. I think that if you were about to have a serious operation or such things, then there could be a case for witholding information for a short time, but other than that , I think it is very important that you are told as and when things happen, good or bad. We implicitly trust our families to keep us informed of things that happen in life. Once you find out that you have had important information withheld for even caring reasons, you would never feel safe or trust them again and could be constantly worrying about what you might or might not know. So do as you would be done by, and tell her. It will be sad news, but obviously not a total shock as she has known about his illness. Also you may be able to give solace to each other. It is YOUR brother and your niece cannot know about the bonds and connections that existed between you all. She is thinking very short term, but your sister has to know the news sometime and this would be the right way to both treat your sister and to respect the family ties between the two of you, without letting your niece possibly cause upset and hurt between you both. Perhaps after the funeral, you might be able to make a visit to your sister soon where you could get together and talk about old times with your brother and grieve together, which I personally feel would be comforting for you both. Hoping that you can resolve the situation in a way that allows you to feel less stressed and calmer