The plainest reading would refer to other Jews, Jesus' kinsmen.... as his brothers. Specifically, the 144K male Jewish virgins that will appear during the Great Tribulation. More generally, as FF Ghost pointed out:
”For whoever does the will of my Father who is in heaven, that one is my brother and sister and mother.”
And what is the Father's will?
In desperation a jailer asked, “Sirs, what must I do to be saved?” Paul and Silas said, “Believe in the Lord Jesus, and you will be saved—you and your household.” And that night, as Paul and Silas shared “the word of the Lord,” salvation came to that jailer’s household.
Whoever gives up their love affair with trusting in religion, good works, government, men in general, and puts trust in Jesus will be saved.
He that trusteth in his own heart is a fool - Pr. 28: 26
Put not your trust in princes, nor in the son of man, in whom there is no help. - Ps. 146 3
The reason the WT has been so successful is because a lot of people really just want someone they can see to tell them what to do. It really is about that simple.