[Uri’el-2012]

But the New Testament is not part of the Septuagint, nor were St. Elizabeth and the Blessed Virgin brothers.

How about GSN-431

Num 36:11; Tob 7:2, Tbs 9:6 or Col 4:10

[Philo-Junius]

Anepsios is used in the Septuagint in the 2 cases you cite, it is true, but the broader use of adelphoi in counterexamples predominates:

Gen 14:14
Gen 14:16
Gen 29:15
Deut. 23:8
2 Kgs. 10:13–14
1 Chr. 23:21–22
Neh. 5:7

And there are further uses of adelphoi, both in the Old and New Testaments in purely figurative sense as well.