I hope you mean sibling sisters, not sisters and brothers, as males and females housed together will still breed, whether they are related or not.
Not all females will lay infertile eggs. Though there are some greater risks for egg binding with infertile eggs, there are ways to help prevent that, by ensuring proper diet, plenty of calcium etc and watching for signs of your dragon being gravid, then providing proper egg laying containers. This doesn't mean the risk is reduced to null but often it is due to people not realizing their dragons can still produce eggs without a male so don't see the signs till it is too late.
Housing a single male with a single female all the time, will mean more stress on that female, as the male will be constantly trying to breed her and she will be laying eggs all the time, which puts alot of stress on her. IF breeding isn't something that isn't wanted, then two females or a single female or male is the way to go. Yes she may lay eggs anyway but she may not and more times than not, they lay infertile eggs later in life than they would with a male around (either in the same cage or nearby). I do have female gargoyle geckos that are dropping infertile eggs regularly and so far had no complications, but i keep a moist egg laying box in with them as well as feed extra calcium. Their eggs look better than many of my fertile eggs from my crested geckos. Of course geckos lay only two egg clutches nearly all the time, so there is a big difference between the numerous eggs a dragon can lay, but proper care can greatly reduce egg binding risks for infertile eggs.
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PHLdyPayne