Luke indicated that Jesus said to pay attention (προσέχετε ἑαυτοῖς). If a brother disciple sinned (ἐὰν ἁμάρτῃ ὁ ἀδελφός σου), they must rebuke the offender (ἐπιτίμησον αὐτῷ). If there is repentance (καὶ ἐὰν μετανοήσῃ), they must forgive him (ἄφες αὐτῷ). This saying about the sinning brother is something similar in Matthew, chapter 18:15, perhaps indicating a Q source, with some minor changes. Luke wanted the one offended to forgive the original offense. Matthew indicated that Jesus said that if one of your brothers had sinned against you (Ἐὰν δὲ ἁμαρτήσῃ ὁ ἀδελφός σου), point out that fault or rebuke him (ὕπαγε ἔλεγξον αὐτὸν), when the two of you are alone (μεταξὺ σοῦ καὶ αὐτοῦ μόνου). If he listened to you (ἐάν σου ἀκούσῃ), you have regained your brother (ἐκέρδησας τὸν ἀδελφόν σου). There was no mention of any forgiveness here. Is this a blood brother or a fellow disciple of Jesus brother? It appears to be a fellow follower of Jesus. Forgiveness was important. Matthew had a more elaborate process of rebuking and forgiving. How do you forgive people?
This is another unique passage of Mark, who tended to point out how these 3 trusted disciples, Peter, James, and John, were confused and still did not understand what was happening. Mark said that the 3 apostles were able to keep this matter quiet among themselves (καὶ τὸν λόγον ἐκράτησαν πρὸς ἑαυτοὺς). However, they questioned or discussed this among themselves (συνζητοῦντες) what the rising from the dead (τί ἐστιν τὸ ἐκ νεκρῶν ἀναστῆναι) was all about.
There is something similar in Mark, chapter 13:1. Jesus came out of the Temple (Καὶ ἐξελθὼν ὁ Ἰησοῦς ἀπὸ τοῦ ἱεροῦ). As he was going away (ἐπορεύετο), his disciples came to him to point out the beautiful Temple buildings (καὶ προσῆλθον οἱ μαθηταὶ αὐτοῦ ἐπιδεῖξαι αὐτῷ τὰς οἰκοδομὰς τοῦ ἱεροῦ). The Jerusalem Temple had been under reconstruction since the time of Herod the Great in 19 BCE but would not have been completed at the time of Jesus.