I thought this was 'Is your sister-in-law with you'?
No that would be, Ydy dy chwaer yng-gnhyfraith gyda ti?
It's the 'gyda chi' that confused me as that's 'with you'
gyda chi is the south walian way of saying "you have" . Sorry I misunderstood as to what you were confused about, but I do think the way of telling the difference is with the word at the start of the sentence.
I'm south walian and didn't recognise it as that. What you're saying about the start of the sentence makes sense now though
Presumably "eich ... gyda chi" or "dy ... gyda ti"?
Suggested adding "Have you got X?" as an alternative answer.