relation-relationship-relative

1 `relation' and `relative'

Your relations or relatives are the members of your family.

• I said that I was a relation of her first husband.

• I'm going to visit some relatives.

The relations between people or groups are the contacts between them and the way they behave towards each other.

• Relations between the two men had not improved.

• Britain has close relations with the US.

2 `relationship'

You can talk in a similar way about the relationship between two people or groups.

• The old relationship between the friends was quickly re-established.

• The French President has shown that he is keen to have a close relationship with Britain.

A relationship is also a close friendship between two people, especially one involving sexual or romantic feelings.

• When the relationship ended two months ago, he was very upset.

RELATIONSHIP VS RELATION (further explanation): 

Both express the idea of a connection between things or persons. Although largely synonymous, the words relation and relationship are not interchangeable:

RELATIONSHIP (link to Cambridge dictionary) tends to be preferred for human connections. It indicates the informal, smaller type of connection between people like close relatives, couples, etc.

 A relationship is a close friendship between two people, especially one involving romantic feelings:

"Relationship" tends to be used more broadly and generally to describe the interactions between specific people or smaller groups of people.  When used for specific people, it often can refer to a romantic connection ("I am in a relationship with him") unless another type of relationship is specified ("her relationship with her coworkers", "the parent-child relationship").  

• "Relationship" can be used in two other ways. It can describe two things and the way in which they are connected:

• Doctors now believe that there may be some relationship/connection between autism and the MMR vaccine.

It can also describe close ties between people or groups of people and the way they feel and behave towards each other:

• The Smiths placed great emphasis on close family relationships and always went on holiday together.

• The relationship between the leaders of the two countries has never been closer.

*When you mean to say "the way in which two things are connected", you could use both "relation" or "relationship". "Relations" is more frequently used in formal writing about countries or large groups of people. "Relationship" is more informal than "relations", but these uses ("the relationship between Iraq and the U.S.") are not uncommon and are idiomatic.

• Next session, we're going to focus more on the relation between Humanism and the works of Shakespeare. ✅ (more formal)

• Next session, we're going to focus more on the relationship between Humanism and the works of Shakespeare.  ✅ (slightly less formal)

RELATION (link to Cambridge dictionary) is used for more abstract connections. It shows the connection between several people, countries, organizations, continents, etc. Note that it is a formal kind of connection. 

"Relations" is used in more formal writing and tends to be used more to refer to the interactions between countries or large groups of people: 

• Relations between Iraq and the U.S.

• Relations between blacks and whites

• Relations between (the leaders of) the two countries have never been closer.

It is typically used in constructions like "diplomatic relations" or "international relations."  Most of the differences are context specific in this sense. For example, we talk about "diplomatic relations" and "race relations", not diplomatic relationships or race relationships

• Diplomatic relations between the two countries were broken off over this incident and their ambassadors were sent home.

• The need to improve race relations in Inner London boroughs is of paramount importance.

Your relations (plural) are also members of your family (=relatives):

• I invited all my friends and relations to my twenty-first birthday party.

• Mark Totterdale and Simon Totterdale (no relation) are both head teachers in Bristol.

Your blood relations are the people who are related to you by birth, not through marriage. If you say that they are your own flesh and blood, you are emphasizing that they are members of your own family:

• He's my own flesh and blood. I can't leave him to fend for himself when he needs my help.