Like was stated earlier, most doms/co doms are bred to normals to produce more of them, this outbreeding helps strengthen the genetics, and keeps it from being inbred. If you buy siblings of super pastels, there is more inbreeding going on, but still unless it is done perpetually with no outbreeding to normals (which is the most profitable way to go for most) Then I wouldnt worry.

If you are concerned about it, just dont buy two lemons you are going to breed from the same person, or dont buy siblings to supers, even though by breeding them to a different pastel from a different person, you would be outbreeding again and lessening the chance of genetic issues.