[jaliberti]

I often see people make this mistake with livebearers. People go to the LFS and select some males and females of the same strain, not keeping in mind, that all the fish in that tank are probably siblings.
Having made that mistake myself with a few fish, I have to admit that deformities were as high as 25% in their offspring. We are talking very good looking fish here (show quality if you wish, with all the horrible things people do to produce those fish).Now I make sure to get fish 2 or 3 months apart if I want a pair.

Those fish 2 or 3 months apart... from the same source... you don't think they are from the same parents? or from the offspring of the same parents?

 

[Nordic]

I think there is less of a chance. When you buy them all at once, there is more of a chance.
Breeders tend to keep more than one breeding pair of a strain, but they go to the wholesalers in batches as they mature.
Even if they were selling offspring of the siblings, they would only be tiny fry at that point in time, if the parents are sexually mature.