They are very similar and their uses inter-laps a lot, but here's an idea of when to use each:
Relationship: It's between one person & another, or a small group. It has more sense of intimacy and familiarity.
Jane and Jake have such a good relationship, I think they will get married soon.
Our families have a strong relationship with each other.
Relation: has less sense of intimacy and familiarity, it lacks a human emotion or it just feels abstract. Or very large groups (countries, big companies)
The relation between negative and positive components of the atom show magnetic features.
The relations between The US and The Middle East showing good progress.