Inbreeding and Line Breeding are touchy subjects for most; but its also a fairly misunderstood topic. Please read the entire post before jumping to any conclusions...
There isn't anything "wrong" with breeding a brother or sister, or breeding a son back to his mother. Yes, its a strange concept for humans to wrap their heads around, and shows like "The Hills Have Eyes" give the majority of people the false impression that inbreeding in a single generation causes extreme neurological, physical, and other mutations. It is possible! But its not always the case... I'll explain.
What inbreeding and line-breeding does it INCREASES the odds of a genetic mutation. Thats why breeders do it - because some mutations are pleasant and can change the phenotype of the animal in a postive way (ie, albinos, red colors, hypomelanistic, etc). But. In doing so you also signficanlty increase the risk of a hidden gene to express itself that has negative effects, such as a deformity.
Here's an example: Lets say that these two bearded dragon are more prone to fatty liver disease due to the way their genes tell them to store fat. This is unnoticable to us physically, and the dragons appear to function fine. In breeding them together, you are passing down their genes to have this issue, and often it continues to compound until a couple of generations later, you have animals that may not be able to survive long due to weak livers and inability to properly process fat.
Most of the time, nothing bad happens. Think about the genepool of most herps in the US - a lot came from just a few specimens that has now exploded into a massive population size. But be aware that you are significantly increasing odds of a negative mutation to surface in forcing only a specific family of genes to be reproducing together.
PERSONALLY - I like to keep things as unrelated as possible and keep the genepool with as much variation as possible. I'm a fan of selective breeding for traits, but I put a higher preference on finding mates that are fairly unrelated. You can breed them together, but be very aware that if any negative defects begin to surface, you need to do the responsible thing and stop breeding these animals, and cull offspring (give away as pets only or humanely euthanize IF they cannot function in such a way that gives them a good quality of life). It's just a safer, more accepted route to breed two animals that are more unrelated, and its worth the investment of another animal to ensure you have good bloodlines. I know the bearded dragon genepool is fairly related as it is, I think I'd prefer to find more unrelated animals, but thats just my opinion...and I'm sure the majority of others as well.
Be sure to read the sticky in the forum and other sources before you even consider breeding as well - its a HUGE commitment with bearded dragons.