"I do not have a sister."
Translation:Chan eil piuthar agam.
There is no agad in I have a sister (tha piuthar agam).
agad means at you, agam means at me
You might want to look at the tips and notes to the lessons – it’s explained there. The tips are available in the browser version of Duolingo (but unfortunately not in the mobile app). You can also view them on the duome.eu website.