when you have more genetic variation, animals (fish) tend to have stronger immune systems. also inbreeding can lead to a lot of birth defects. and chances are if you get the fish from one source those fish have already come from a common gene pool. you can breed them together but its suggested that after a while you introduce more from different sources for more genetic variation.
Breeding siblings for one or two generations isn't really a big deal. The inbreeding is easily corrected with one generation of outbreeding to an unrelated fish. The danger occurs when you breed several generations of siblings, so that you have fish getting two copies of defective genes and ending up with mutations that can cause physical deformities and health problems. The same concept applies to every species that produces sexually, including humans, dogs, cows, etc. Genetic variation corrects this, because with many of these mutations, if a healthy copy of the gene exists, it will override the defective copy.
Yep, if you get all of your fish from the same store (at the same time) then you'll likely end up with a small genetic pool. As mentioned above by several people, the smaller the genetic pool you have the more likely genetic defects will become amplified....leading to unhealthy fish or at least fish that are more sensitive/suseptible to disease.
I think it's something like 5 or 6 generations of inbreeding before you start getting an increase in negative effects (that is for fish)(But you don't know how much inbreeding has gone into those angels at the store either). So yea if you are just looking to get a pair it probably doesn't matter if they are from the same family, but if your going to do a lot of breeding down the road, try to get stock from multiple unique sources.