If your family has lived in the US for a while, search for them in each available census, starting in 1930 and going backwards. Find your grandparents, find them as children with their parents, find their parents as children with their parents and that will give you siblings. This isn't an easy task, but it's the best way to track someone. It helps if they have a relatively uncommon name, and you know the state and hopefully the county where they were living at the time of the Census. This will give you at least an approximate date of birth (including month and year in 1900 Census). You can use this info to find birth records if they exist, which will help you identify them in death indexes.