(Nay, but your own selves have beguiled you into something. So patience is most fitting (for me).) Muhammad bin Ishaq said, "When they went back to Ya`qub and told him what happened, he did not believe them and thought that this was a repetition of what they did to Yusuf. So he said,
(Nay, but your own selves have beguiled you into something. So patience is most fitting (for me).) Some said that since this new development came after what they did before to Yusuf, they were given the same judgement to this later incident that was given to them when they did what they did to Yusuf. Therefore, Ya`qub's statement here is befitting,
When the sons of Sayyidna Ya` qub (علیہ السلام) saw the extreme suffering and patience of their father, they said: قَالُوا تَاللَّـهِ تَفْتَأُ تَذْكُرُ يُوسُفَ (By God, you will not stop remembering Yusuf ...), meaning thereby that every shock ends, after all, and so does every sorrow. The passage of days in life makes one forget them. But, he continues to be where he was, even after the passage of such a long time with his sorrow being as fresh as when it came.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 83–86
By saying this Jacob exposed the evil in the hearts of Joseph’s brothers. When they left their father, they had taken Benjamin with them, promising his complete safety; and when a cup was discovered among Benjamin’s belongings, they could not even say in his defence that just the finding of the cup did not prove him to be a thief. Perhaps somebody had put it in his baggage; or it might have been packed up in his baggage by mistake. Instead, they confirmed the Egyptians’ suspicion about Benjamin’s alleged guilt by their referring to a theft formerly committed by his brother.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.