"Do you have a sister-in-law?"
Translation:Oes chwaer yng nghyfraith gyda chi?
As far as I know, efo means "with" in the north while gyda means "with" in the south -- but while gyda is used in the south for "have" constructions, efo is not so used in the north.
Instead, the options are Mae X gyda (fi, etc.) (south) and Mae (gen i, etc.) X for "(I, etc.) have an X". Which use not only a different word (invariant gyda in the south, inflected gan in the north) but also a different word order (gyda after the possessed noun in the south, gan before the possessed noun in the north).
So I think Oes efo chi ...? is not possible, only Oes gennych chi ...?.