If the cousin belongs to one of the father’s siblings, then the person is second cousins with that cousin. Descent is used to identify family relationships, starting with the first shared ancestor or relative.
People born to siblings are considered cousins. The grandparents (the parents of a set of cousins’ parents) are the next set of family members to be added to the family tree because those siblings are independent sets of people.
They are second cousins because the child of a sibling and that sibling’s cousin must move backward two places to discover a common ancestor between the cousin’s grandparents and the child of the sibling’s great grandparents.