Interbreeding between monitors and most reptiles is normal. All the ackies here in the US have pretty much the same ancestry and have been breeding mother-father-sister-brother for many generations. The benefit of having hatchlings raised together is that they are more likely (not definitely) to get along.
That being said, interbreeding is not necessarily a good thing as you lose the gene variables (differing traits) of having two unrelated (or distantly related) monitors. If mom and dad (who are brother and sister) come from parents who both have a trait that makes them more susceptible to disease then children are more likely to have that same trait (on down the line for generations).
But what the dealer told you about getting all males is not true either. I am assuming they are referring to the the observation that some species of monitors tend to sex themselves after hatching in a manner that produces a nice balance of male versus female (ie 4 hatchlings will be 1.3 or 2.2) and some people would assume by that that solitairy animals would likely be male.
I don't believe that to be true (the solo animal being male only). The ratio part of group hatchlings might hold true, but I haven't seen any fact to back that up as being an absolute. From what I have read, with a solo animal you are going to be looking at 50/50 chance of either sex most likely.
Hope I didn't confuse the subject more and good luck.
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-Andrew
