I think that you are looking for a question word that doesn't exist.
It would be convenient if the English language had an ordinal question word to correspond to 'first', 'second', 'third' and so on - a word like 'Whatth' of 'Whichth' - but unfortunately, it doesn't.
Here are some examples of conversations that can never happen:
My birthday's in April.
April the whatth?
I've got four sisters.
Me, too. I'm the eldest. What about you? You're the whichth in your family?
I'm the second.
These handy ordinal question words have yet to be invented, so we just have to ask in other ways, such as 'Where do you come in your family?' or 'Where's your bike in the line?' or (more simply) 'Which one is your bike?' and then you'd just answer 'The third from the left'. There are always ways of getting round these gaps in the language.