A friend of mine asked me for the translation of "sisters" in Scots Gaelic? I told her it was at least along the lines of "Phiuthar" but that I'd get back to her. Any help would be appreciated!!

"Sisters" is "peathraichean" in Scots Gaelic.
"Phiuthar" is "(one) sister" with a consonant change at the beginning (can't happen if there's no word before)

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

My friend and her sisters want it for a tattoo. Would peathraichean be enough, or would you need an article before?

Yes, peathraichean by itself means "sisters". If you wanted "the sisters", that would be na peathraichean, and "of the sisters" would be nam peathraichean. I'm mentioning the latter two forms in case you've seen them somewhere and wondered about the difference.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Thank you! I'll tell her