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.
I incline to the step-brother theory, myself, but the key point in my mind is that a precision is being imposed upon “adelphos” which it simply didn’t have in koine.