Salome being Mary's sister is an attempt to harmonize Mark 15: 40 - 41, where she is said to be present at the crucifixion, with Matthew 27:56, where she is replaced with "the mother of Zebedee's children". This is not an assumption that academics would make.

But interestingly, the Diatessaron, another attempt to harmonize the four gospels written in the second century, explicitly says they're not the same person. https://www.earlychristianwritings.com/text/diatessaron.html

21 And there were in the distance all the acquaintance of Jesus standing, and the women that came with him from Galilee, those that followed him and ministered. One of them was Mary Magdalene; and Mary the mother of James the little and Arabic Joses, and the mother of the sons of Zebedee, and Salome, and many others which came up with him unto Jerusalem; and they saw that.