Jews refer to all other Jews, whether believers or not, as brethren.
Recalled what Jesus said in Matthew
Matthew 21:43 Therefore say I unto you, The kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof.
Peter is urging the Jewish brothers, who are still rejecting Jesus as their Messiah, to repent and be water baptized, so that they can join the little flock.
It is in line with his appeal in Acts 3
22 For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.
23 And it shall come to pass, that every soul, which will not hear that prophet, shall be destroyed from among the people.
24 Yea, and all the prophets from Samuel and those that follow after, as many as have spoken, have likewise foretold of these days.
25 Ye are the children of the prophets, and of the covenant which God made with our fathers, saying unto Abraham, And in thy seed shall all the kindreds of the earth be blessed.
26 Unto you first God, having raised up his Son Jesus, sent him to bless you, in turning away every one of you from his iniquities.