I looked these ones up in the Oxford Concise English dictionary. Relative is defined as “considered in relation or in proportion to something else”.
Relation is defined as the way in which two or more people or things are connected or related.
I see this as relative being about a comparison, and relation about a similarity.
So, you can say, “the relation between rain and snow is that they are both wet and cold; relative to snow, rain is wetter but less cold”.
As regards the other main meaning, a relative is a person connected to another by marriage or blood (I’d add or adoption or arrangement there to cover adoptions, step-families, etc.) and a relation is defined simply as “a relative”. Your sister is your relation, and also your relative.
Always nice to have some subtlety on a Monday …
You can find more troublesome pairs here.