Did you miss the posts about language?
The use of brothers is substituted for cousins.
Even Paul writes to the different areas/communities as “brothers.”
I stated that in my post which you must not have read. The theory is that the term brothers meant cousins. No sale here but they can believe what they wish. All we really know is what is plainly stated in the Bible and anything beyond that is speculation. And even then there are varying interpretations of some passages .. so it goes on and on.