It's impossible to tell? That's not right. Look closely at their dresses then look at their sisters' dresses. There are differences.
They are not girls either? They have skirts like their sisters but that does not mean that they are definitely girls.
The correct answer is that they are probably little boys. If you look at the bodice (body) of one of their dresses you can see that it looks more like their father's than their sisters'. Boys wore clothes like this until they were about seven years old.