I may be wrong about what a relative minor is.. but here goes..

Ok.. so the relative minor scale would be the scale the pentatonic minor scale you would use, is created from. The most commonly used scale in modern day blues-derived music is the pentatonic minor scale. How do you find which pentatonic minor scale to play for a particular key? Easy.. you play the relative minor, otherwise known as the Aeolian mode.

Let's take the C Major scale:
C D E F G A B
1 2 3 4 5 6 7

The Aeolian mode is the 6th mode, so we take the 6th note from up there.. A. Now we play all the same notes but starting from A and ending on A:
A B C D E F G

This is the A minor scale.. and also the A Aeolian scale.. and C Major's relative minor scale I suppose.

I am currently playing a piece of music using the D Major scale.. so let's have a look at that one:
D E F# G A B C#
1 2 3 4 5 6 7

6th mode... B Aeolian.. So I can easily play the B minor pentatonic shape and then fool around there.. If you're ever stuck with what to play for a solo, the pentatonic minor is a life saver.. and if you know which minor to play for which key, it makes it easy to "centre" yourself again after a mistake..