However, I have a little new light to cast on this. I have often wondered what the translators of the 16th and 17th century really thought about words like αδελφος. Now I have a least a glimmer of an idea.
At Christmas my aunt gave me a 1654 Christianus Schotanus Greek - Latin lexicon. Here are two entries in it.
αδελφος - frater, cognatus, consanguinis, Christianus, proximus, popularis
αδελφη - soror, Christiana, uxor, cognata
For what its worth! Fascinating, isn't it? Your neighbour is your brother and your wife is your sister. And by that same token, why isn't your wife your neighbour?
In any case, you must notice that there is a certain latitude, a breadth of interpretation allowing discretion in translation. We must recall those good old days.