filial is not a very commonly used word, so many people would not even understand the question, I think.
And even if they do understand the word, rank is not very specific -- there are many criteria that could be used to rank siblings, and birth order is not likely to be the first one that comes to mind.
In my experience, people don't usually ask this question directly, it's more often asked indirectly, as in:
Are you the oldest child?
Do you have any older brothers and sisters?
When answering, they'll normally give their actual position in the response.