In some situations, both can be used. But your relations are your family members, e.g. mum, dad, brother, sister-in-law. Your relationship is your connection between someone, often romantic. "We are (not) in a relationship." = Romantic "We have a platonic relationship." = We are friends. "I have a good relationship with my brother, but a bad relationship with his wife." = My brother and I like each other and talk often, but his wife and I do not like each other; here, relationship refers to the feeling/connection. Both the brother and wife are relations. "They are her relations." = Family members. (Note: "relations" is unusual, "relatives" would be more common.) This does not mean you like them. We also have the phrases: "This bears no relationship to (b)" = There is no connection between this and (b) "In relation to (b),..." = Connected to (b),...