Another reason for this reading, beyond the immediate context and how the author uses "kai" in the verse itself, is that this reading is consistent with how the author likes to use "kai" in lists generally.
John 2:11 is an example where each item in the list also gets separated by a "kai": Jesus (he), "and" his mother, "and" the brothers, "and" his disciples, went down to Capernaum.
Μετὰ τοῦτο κατέβη εἰς Καφαρναοὺμ αὐτὸς καὶ ἡ μήτηρ αὐτοῦ καὶ οἱ ἀδελφοὶ καὶ οἱ μαθηταὶ αὐτοῦ
John 2:14, just a few verses later, similarly--Jesus finds people in the temple selling cattle "and" sheep "and" doves.
καὶ εὗρεν ἐν τῷ ἱερῷ τοὺς πωλοῦντας βόας καὶ πρόβατα καὶ περιστερὰς καὶ τοὺς κερματιστὰς καθημένους
It also comports with how we see the author uses postpositive adjective phrases to describe people or things he mentions. So John 1:40, "Andrew, the brother of Simon Peter, was one of the two . . . ."
Ἦν Ἀνδρέας ὁ ἀδελφὸς Σίμωνος Πέτρου εἷς ἐκ τῶν δύο τῶν ἀκουσάντων παρὰ Ἰωάνου καὶ ἀκολουθησάντων αὐτῷ
Or John 6:1, Jesus went over the "Sea of Galiliee, [that is, the Sea] of Tiberias."
Μετὰ ταῦτα ἀπῆλθεν ὁ Ἰησοῦς πέραν τῆς θαλάσσης τῆς Γαλιλαίας τῆς Τιβεριάδος
Or John 6:8, similar to John 1:40, another mention of "Andrew, the brother of Simon Peter."
λέγει αὐτῷ εἷς ἐκ τῶν μαθητῶν αὐτοῦ, Ἀνδρέας ὁ ἀδελφὸς Σίμωνος Πέτρου
Or John 6:42, "isn't this Jesus, the son of Joseph . . . ?"
καὶ ἔλεγον Οὐχ οὗτός ἐστιν Ἰησοῦς ὁ υἱὸς Ἰωσήφ, οὗ ἡμεῖς οἴδαμεν τὸν πατέρα καὶ τὴν μητέρα; πῶς νῦν λέγει ὅτι Ἐκ τοῦ οὐρανοῦ καταβέβηκα
Or John 6:71, "now he was speaking of Judas, [son of] Simon Iscariot."
ἔλεγεν δὲ τὸν Ἰούδαν Σίμωνος Ἰσκαριώτου· οὗτος γὰρ ἔμελλεν παραδιδόναι αὐτόν, εἷς ἐκ τῶν δώδεκα.
So while it's certainly not necessary to the dogma of the perpetual virginity to find in John 19:25 a use of "sister" to speak of someone who was not actually a biological sibling of the mother of Jesus; and we need not be dogmatic about how to read the verse, this is why I stand by why, correctly read, it indeed refers to three people--the mother of Jesus, her sister Mary of Clopas, and Mary Magdalene.
And for what its worth, not Hahn or Bergsma directly, but I did find this article from their organization, the St. Paul Center, expressing this view:
Who Were the Women at the Crucifixion? - St. Paul Center