Relatives belong to the same family. There are close relatives such as siblings, parents or own children. We also call this the nuclear family. The extended family also includes more distant relatives such as uncles and aunts, cousins and grandparents.
Where the border between the nuclear family and the extended family lies depends very much on the relationships between the relatives. It also depends on the country and culture you live in. So grandparents can be counted as immediate family or not.
A distinction is also made between blood relatives and other relatives. Blood relatives are parents with their children or siblings among themselves because their bodies have many of the same characteristics. It used to be said: "They have the same blood". Parents have a special relationship with their children, also special rights and duties. That is why they have their own article in the Klexikon.
Marriage does not make a woman and a man blood relatives, even if they have children together. The parents still have "different blood". It is different if a man and a woman have always been related because they belong to the same family. In many countries it is forbidden for parents to be closely related. This is because deformities can then occur in children born together.
Another difference is this: the man who fathered a child is the biological father. This is true even if the child does not know him. He could have died, disappeared or whatever. But maybe the mother is then living in a new relationship. If this man takes good care of the child, then he is a kind of "substitute father". This is also called "the social father". If he adopts the child, it is considered his own in the eyes of the law.