relative can have different meaning dependent on context.
So a brother and sister are relatives.
However from your question, I think you are thinking of relative in its comparative use. Here relative would compare two objects to some other item.
Most birds are smaller than people, but relative to ants, birds are giants.
Relevant means "pertaining to" , this means that the association has meaning.
So relevant evidence in a criminal trial would be being found at the scene of a crime. The opposite word is irrelevant. Having coffee for breakfast would be irrelevant in the same criminal trial.