how do you say following in english?

Okay.. Obama is the 43rd President of America. This is what we say, because we number them in order from the first one. The point is: they are all presidents. And it wouldn't matter if some had been females by now. (That just hasn't happened in history so far.) It would still just be 'one category' ...of 'president'. They are all presidents.

For a boy among siblings: We can think of 'siblings' as a category in several ways. Siblings might be all boys. They might be all girls. They might be some boys and some girls. So with siblings, you *could* say: 'the oldest boy', or 'the second oldest boy' (etc.) or 'the youngest boy'. The girls could be described similarly using 'girl' instead of boy.

IF we don't consider whether they are boys or girls (it doesn't matter, in other words), then we could say 'the oldest sibling', 'the second oldest sibling', 'the third oldest sibling' (we are working down from oldest to youngest this way). Or we could say 'the youngest sibling', 'the second youngest sibling' (etc.) (and we would be working up from the youngest to oldest).

But to say 'the 7th' (or 'the 43rd' -- can you imagine a family that big?!) sibling doesn't work the same as with Presidents. Because we don't traditionally name siblings in the order in which they came -- there's no 'tradition' with that. They can be numbered from oldest to youngest, or the opposite order. OR you might want to consider the boys and girls as separate sub-categories with siblings .. when you describe if they are '1st' or '2nd' or '3rd.' ...

Hope this is fairly clear. There's no automatic order with siblings, that's all.

I think I understand what you mean. Hope this website might help: http://english.stackexchange.com/questions/21876/h...