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I dont know about them being brothers and sisters, but around here alot of the stores sell "mixed cichlids", which usually end up interbreeding and you dont get a pure strain of anything. I would say the chances of them being related are quite good. Fish that are prolific spawners probably are haviing babies while they are being transported to the store.
If you want to breed cichlids buy them from 2 different store to ensure different genetics. Or order especially for breeding. You might pay more but a good LFS should accommodate your needs.
I think interbreeding is the reason they come up with all these weird morphs that you end up paying big bucks for. Just look at the clownfish in saltwater.
You more then likely would not see any recessive gene affects as the gene pool could be broad enough to prevent problems in the F1 offspring. When you get into the F2 and F3... offspring then you may get into genetic problems, some of which you might like. In the wild these offspring don''t stand a chance of surviving because they stand out and become easy prey. In controlled breeding the predation factor is eliminated and all genetically weak offspring have a very good chance of surviving.