All animal kingdoms do it; incest. You never know if some of the human population does it....but if it happens, it's definitely Taboo!!! As a result, you will have weaker fries. Just think about breeding Electric Blue Jack Dempsey!!! The blue jack dempsey are usually males and is bread w/ a regular jack dempsey female. The fries has the genes for regualr and blue genes. They take the female fries and bread it back to the father to get 50% regular and 50% blue jack dempsey. This causes some serious issues as you will find out that not many will live up to adulthood due to the weakend genes!!! I know a guy who breeds them and has informed me not to buy any EBJD under 1.5-2"....If you want one, buy the~4-5"...they have passed the death genes.
Hence, yes..brother and sis can breed..outcome depends? As to what length they start breeding, sometimes they start early ~3-4" and sometimes larger. They will pair up if you put a group together..I breed them and it has takened me two long and expensive year..check what I have bread in the breeding section!!! Also, fertile or not, the eggs will be white w/in 24hrs of spawning if it not fertile...