A "Leader" is the person who leads. e.g. He is the leader of the organisation.
"Leadership" is the state or position of being a leader. e.g. The organisation thrived under his leadership.
"Relation" means the way in which two or more things are connected. e.g The relation between the farmer and the land.
"Relation" can also mean a person who is in the same family. e.g.
A distant relation of mine.
Here, relation is synonymous with relative.
"Relations" means the way in which two people, groups or countries behave towards each other or deal with each other. e.g. Diplomatic relations between two countries.
you can use relationship in places where you use relation or relations. But, relationship is not used to denote a relative.