I couldn't help but think of the turrets from Portal with this sentence. Sometimes they ask "Are you still there?" whenever you drop out of their sight.
"there" and "yet" are ambiguous in English - "he is there yet" and "he is there still" both mean the same thing, but *is he there yet?" and "is he there still?" are usually interpreted differently (hasn't arrived vs hasn't left), though they can be both interpreted as (hasn't left).
That ambiguity makes it difficult to translate questions with fós in them - does An bhfuil sé ag cur báistí fos? mean "Is it raining still?" or "Is it raining yet?" ?
In English, the position of the word "still" can change - you can say "Are you there still?" or "Are you still there?". The convention in Irish seems to be that if you leave the fós at the end of the question, you can translate it as "yet", but if you move it into the middle, as in English, you can interpret it as "still".
An bhfuil sé ag cur báistí fós? - "Is it raining yet"
An bhfuil sé fós ag cur báistí? - "is it still raining?"
An bhfuil tú ann fós? - "Are you there yet?" (haven't arrived) An bhfuil tú fós ann - "Are you there still?" (haven't left)
There is no reason in this case - ann can (usually) be used where you would use ansin, (except when ansin means "then"!).Ansin can imply a more specific location than ann.
(I don't know if there's a dialect issue, where some dialects tend to prefer ann over ansin).