It has been found in writing concerning the Spartans and the Jews that they are brethren and are of the family of Abraham: This verse refers to some supposedly old document the Spartans claim to have discovered. The document claims that both the Spartans and the Jews descend from Abraham (see the comments on verse 6). Good News Bible provides a good model here.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.