soror

Soror, sororis, f., third declension, and one of the oldest words in the language: the same Indo-European root as English sister, German Schwester and Sanskrit svasar. Frater, mater and pater are its neighbours in that family, which is why they all decline alike and all keep the -r.

In the monastic corpus soror is the nun, as frater is the monk, so sorores in a charter usually means a religious community rather than anybody's siblings, and reading it as family will misdate the document. Derivatives: sororius, "of a sister", and the grim sororicida. French kept soeur and Italian sorella, but Spanish replaced it entirely with hermana, from germana, "full-blooded".