just had a classic image in my head of all these mcdonalds fries swimming around my tanks hahahahaha
there might be a noticable difference. The thing is you have no idea how inbred your particualr fish are already. if you have two wild caught siblings then you could prob get away with a few generations of inbreeding, but most aquarium stock are already heavily inbred meaning that you risk loosing the whole lot.
with siblings there is a, conservativly, 1/8 chance they will end up with the same copy of a gene. doesnt sound too bad but when you consider there are 1000s of genes that make up a fish and it only take two copies of a single bad gene to kill a fry then you can see why it has become such an issue.
crossing cousins means the chances go out to 1/16 of getting the same gene.
there is no gaurenteed level of relatedness that will produce viable offspring. sometimes you get unlucky and two totally unrealted parents will be genetically uncompatable, just like in humans
your safest bet is to see if you can swap on of the breeders over. I know that I do this on a regular basis. I will often swap one of my experences breeders with an younger male. People seem to like this as they get a proven breeder and i get some fresh meet to train up and we both get a new bloodline.