Jesus is not ashamed to call them brothers, those who do the will of God.
Heb 2:11 For both the one who sanctifies and those who are being sanctified all have the same Father. That is why Jesus is not ashamed to call them brothers [The word for “brothers” here and where context allows may also be correctly translated “brothers and sisters” or “siblings]. – This could easily conclude that the brothers are limited to a small number of saints. However, this should not be deduced. Matthias and Mark are expanding their group of brothers and sisters: Mar 3:35 (and Mat. 12:50) For whoever does the will of God is my brother and sister and mother.
The Bible speaks of the fact that, overall, there is only one group. John says: 10:16 I have other sheep, which are not of this fold. I must bring them also, and they will hear my voice. They will become one flock with one shepherd. – They all are going to be children of God. The believers receive the Spirit and seal of God and salvation during the coming time of tribulation. Rom 8:16 The Spirit himself testifies with our spirit that we are children of God; 2 Cor 1:22 who has placed his seal on us and has given us the Spirit in our hearts as a down payment. 2 Cor 5:5 Now he who made us for this very thing is God, who also gave to us the down payment of the Spirit. NKJV … “who also has given us the Spirit as a guarantee.”