According to J. Ramsey Michaels, in his NICNT commentary on the Gospel of John, John 20:17b is a milestone in the Gospel, for it is the first and only instance (out of 120 in all!) in which God is explicitly identified as "Father" of anyone except Jesus himself. So you brought out a good point. According to the commentary, up to that point, "disciples" are who believed in him (2:11), while his "brothers" did not (7:5). But in John 20:17b there is an abrupt change; now the term "brothers" refers to the disciples. So in John 20:17 Jesus made the pronouncement of elevating humankind to being able to address God directly as "Father", as Jesus has done. Therefore, it is not simply an appellative.
J. Ramsey Michaels' translation of John 20:17:
Jesus says to her, “Don’t take hold of me, for I have not yet gone up to my Father. But go to my brothers and say to them, “I am going up to my Father and your Father, and my God and your God.”
Full quote of the commentary on John 20:17b (emphasis mine):
[Jesus] has “not yet gone up to the Father,” but now he is “going up to my Father.” It is unclear whether the present tense actually refers to something in the near or immediate future, like “going” or “going off” in the farewell discourse, or whether the process of “going up” has in some sense already begun.
Whichever it is, it involves a change in his relationship to the Father, a change involving the disciples as well. Up to now, Jesus’ “brothers” and his “disciples” have been clearly distinguished from one another (see 2:12). His disciples “believed in him” (2:11), while his brothers did not (7:5). Yet now, abruptly, the term “brothers” refers to the disciples, for it is to them that Mary will deliver the message (v. 18). Once again, the statement recalls the risen Jesus’ command to Mary Magdalene and “the other Mary” in Matthew, “Don’t be afraid. Go tell my brothers that they should go away into Galilee, and there they will see me” (Mt 28:10), yet with significant differences. Here, instead of “Don’t be afraid,” which is more or less expected (after 28:5), we have “Don’t take hold of me,” which is quite unexpected. Also, instead of promising a visit to Galilee, where the disciples will see him, he merely confirms that he is going away—more specifically, “going up” to the Father—and makes no promise about seeing him again. Yet the two stories have in common the sending of the woman (or women) to the disciples with a message, and—more remarkably—Jesus’ reference to his male disciples as “my brothers.” Matthew offers no explanation for the abrupt change in terminology, for the angel at the tomb had referred previously to “his disciples” (v. 7). At most, the shift could be inferred (if the reader had a good memory) from Mt 12:49, where Jesus “pointed his hand at his disciples and said, ‘Look, my mother and my brothers!’ ” John’s Gospel, however, provides a definition of “brothers” in the immediate context, for Mary is to “say to them, ‘I am going up to my Father and your Father, and my God and your God’ ” (v. 17b). Jesus’ disciples are his “brothers” in that they have the same “Father” in heaven, the God of Israel whom they worship. One of them, as we have seen (19:27), even has the same mother! This is a milestone in the Gospel, for it is the first and only instance (out of 120 in all!) in which God is explicitly identified as “Father” of anyone except Jesus himself. Once or twice Jesus has come close to such an identification, as when he told the Samaritan woman of a day “when the true worshipers will worship the Father in Spirit and truth” (4:23), or when he called the disciples his “friends” (15:14–15), and reminded them that “the Father himself loves you, because you have loved me” (16:27; also 14:21, 23). Still, it has always been either “the Father” or “my Father,” never until now “your Father”—this despite the designation of believers as “children of God” (1:13; 11:52). It is almost the exact opposite of the other Gospels, notably Matthew and most notably the Sermon on the Mount, where Jesus begins by referring to God over and over again (beginning in Mt 5:16) as “Your Father,” or “Your Father in heaven,” and only near the end discloses that the key to it all is “doing the will of my Father who is in heaven” (Mt 7:21; see also Mt 11:27). In John’s Gospel, by contrast, “the Father” is Jesus’ Father first of all, and only by virtue of his resurrection the Father of those who believe.
The question remains, Is the message Mary is to deliver a message for her as well? Is she included among those to whom Jesus says, “I am going up to my Father and your Father, and my God and your God”? If the male disciples are his “brothers,” is she numbered among his “sisters”? What does the message mean for the messenger? Perhaps the answer depends in part on whether Jesus is using direct or indirect discourse. It is a matter of definition. Strictly speaking, he is using direct discourse, in that “I am going up to my Father and your Father, and my God and your God,” is exactly what he wants to tell his disciples. Yet it is indirect, in that Mary will not use just those words, and those alone. She will not presume to speak for Jesus in the first person, like a prophet speaking for God. Rather, she will speak for herself, referring to him in the third person as she repeats what he told her to say: either “Jesus is going up to his Father and your Father,” and so on, or (more likely) “Jesus said to me, ‘I am going up to my Father and your Father.…’ ” The words are spoken, after all, first to Mary, representing all who believe or would believe, male or female—not least the presumably male and female readers of the Gospel! It is Mary’s job to pass the message along in her own words to the male disciples, wherever they might be, thereby making them the first of Jesus’ honorary “brothers.” If the pronouncement elevates humankind to the point of being able to address God directly as “Father,” even as Jesus has done, it confirms at the same time Jesus’ humanity, to the point of worshiping “my God and your God” as any human being might do, even though he has been introduced from the very beginning of the Gospel as himself “God” (see 1:1, 18).
