Use the Cousin within Generations term to create a filter where people are selected if they are a descendant of one of the ancestors of the subject person within the generation you specify.
For example, if you choose yourself as the subject, and set the Generations property to 1, Cousins within Generations will select your parents and their descendants. That includes your siblings, but also includes any grandchildren of your parents, any great-grandchildren, etc.
If the subject person does not have an ancestor in a specif line at the generation depth you specify, the furthest ancestor in that line is selected instead.
For example, if you choose yourself as the subject, and set the Generations property to 3, Cousins within Generations will select your great-grandparents and their descendants. However, if you have not identified the parents of one of your grandparents, Cousins within Generations will select the descendants of the grandparent whose parents are unknown.
See the Filters help page for more information about filters.