Inbreeding accentuates whatever genes are already in the bloodline. It makes it more likely that the same genes - good or bad - will find each other and express themselves in the next generation. Inbreeding is a common way to reproduce a new recessive trait when only one animal is available that carries it. For example, the first amel. corn was bred to a normal, and then the babies back to each other or to the amel. parent to produce more amels.
In most cases, there won't be noticeable deleterious genes expressed in the first generation. Continued inbreeding over several generations to increase the expression of a "certain look" in okeetees, sunglows, etc, may also increase the expression of unwanted genes as time goes on. So very careful attention to detail, and careful culling of future breeders must be followed in order to avoid introducing problems.
It is easier for most people to just avoid inbreeding unless there is a reason such as the ones mentioned above, or if sibs are the only available choice, and you plan to outcross the next generation. You should also plan to avoid breeding the sibs again if genetic problems show up in early pairings (not likely though). If you are purchasing from a good breeder, they should have an idea if a particular bloodline typically produces problem babies. Then you will have a good idea whether or not that line should be outcrossed.