By comparing Matt. 27:56 with John 19:25, it seems likely that Salome was sister to Mary (Jesus’ mother).
“By inference from the other Gospel accounts, Salome is also identified with the mother of the sons of Zebedee (Mt. 27:56) and with the sister of Mary the mother of Jesus…. (John 19:25)… If these identifications are valid, Salome was Jesus’ aunt and James and John were His cousins. This would help to explain her request that Jesus give them preferential treatment (Mt. 20:20-24; Mark 10:35-40), as well as Jesus’ request of John at the cross (Jn. 19:26f.) and the immediate response of James and John to Jesus’ call (Mk. 1:19f.).” [“Salome,” International Bible Encyclopedia, Volume IV, p. 286].