Evidently Peter and another disciple had followed the arresting party from Gethsemane back into Jerusalem to the high priests" palace (Gr. aule, "court" or "courtyard," cf10:16). [source][source][source]
Traditionally commentators have understood the "other disciple" to have been John , the "beloved disciple" (cf13:23; 19:26-27; 20:2-9; 21:1 , 20-23 , 24-25). However because John described this "other disciple" as someone who had a close relationship with the high priest (Gr. gnostos, cf. 2 Kings 10:11; Psalm 55:13; Luke 2:44) many modern interpreters question the traditional view. It has seemed incredible to some of them that a fisherman from Galilee would have had the close relationship with the high priest (i.e, Caiaphas, John 18:13) that this passage presents. Nevertheless it is entirely possible that John as the son of a supposedly prosperous fisherman (cf. Mark 1:19-20) did indeed have such a relationship. [source][source][source]
"Salome, the mother of John , was a sister of Mary, Jesus" mother (cf. John 19:25 with Mark 15:40), and would have been equally related to Elizabeth, whose husband, Zechariah , was a priest ( Luke 1:36)" [1][source]
Moreover the New Testament presents Peter and John as having the close relationship that this passage describes (e.g, 13:23-24; 20:2-10; 21:20-24; Acts 3:1; Acts 3:11; Acts 4:13; et al.). Therefore the traditional view may be correct. [2] The correct identification of the "other disciple" is not essential to a correct interpretation of the events, however. [source][source][source]