If you assume that the siblings can be differentiated (thus ordered), it makes no difference. You can for example order them taking their names alphabetically, or by weight, or by location at a given time, etc...

If differentiation is not possible, then they are the exact same person [1], so the parents have one child and it must be boy then ;)

[1] https://en.wikipedia.org/wiki/Identity_of_indiscernibles