Inbreeding in any animal is not recommended because it does increase the chance of deformities and illness by weakening the gene pool. Keep in mind, captive axolotls are already the epitome of inbred, as they have been so interbred to produce specific colour lines. The rate of deformities is particularly high in the species anyway for this reason. You can look at it as though you are not doing that much damage because it is only one line and they are as inbred as they can possibly get anyway, but understand that the risk is even further heightened in a brother/sister cross because the more inbreeding occurs, the more chance there is for deformities per embryo. So there is the brother sister danger on top of the inbred species deformity chance. Most of my 208 larvae from an unrelated cross has some sort of problem. An odd number of gills, different gill directions, stumpy toes, odd number of toes, some can even have no eyes. Some consider this one of the charms of axolotls, they can be so weird, be it from inbreeding or not.
Also there is the moral dilemma of the family thing. Also you can argue that it harms the captive population by inbreeding, continually making it weaker by preventing natural selection. It’s a choice that is up to you, many keepers interbreed for specific colours, then outbreed again. Because your colours are not rare, you could always go pick up another light coloured, not related(ish) axolotl from the pet store or a breeder? Just to mix the gene pool up a bit.
And you would be surprised. Because 'abnormal' colours like albino or leucistic have been so heavily bread for in the population most dark colours carry those genes as well. Your darker axies are most probably carrying light genes and if you bread one of those with your light axolotls, a majority of the babies will probably be light too. And it avoids the brother/sister cross.
good luck
so i can get a bit of both axies 
