I don't tell my friend her sister died
Why not? This is serious information she needs to know.
I don't want to hurt her feelings.
I know that you do not want to hurt the feelings of your friend. But that is a serious thing and I think the truth should come out.
I'm not sure what to do. Maybe I should tell her?
This is her sister we're talking about. She has every right to know what happened to her. You keeping that from her is wrong.