Do you have a sister?
A bheil piuthar agad?
Look at the conversation again.

A bheil piuthar agad?
Do you have a sister?

Tha dithis agam. Cia mheud bràthair no piuthar a th’ agaibh?
I have two. How many brothers or sisters do you have?

Tha bràthair agam. Cia mheud seanair a th’ agad?
I have a brother. How many grandfathers do you have?

Seanair? Tha aon seanair agam.
Grandfather? I have one grandfather.