In New Testament usage, depending on the context, the plural Greek word adelphoi (translated "brothers") may refer either to brothers or to brothers and ­sisters." We will always have arguments about translations and meanings, The Tablet concludes. Source. The Tablet; Image: Rorate Caeli
So, the plural adelphoi in Greek can mean "male siblings" or "siblings," depending on the context. Since Paul addresses male and female adelphoi in his letters, then a perfectly proper and literal translation in English is siblings or, less clunkily, brothers and sisters .