Peathraichean is the plural. It is used exactly as you would sisters in English, except after dà and a few other words that will be introduced in due course.
Dà is followed by the singular (or something that looks like it to be technical) and causes lenition, so it is dà phiuthar. To avoid confusion it is much better to think of the two problems separately:
Should it be lenited?
Should I use the plural form?
Especially when doing the questions where you have to pick the right one from a list of similar words, it is much better to answer these questions separately.