(טז) וַיְצַוּ֕וּ אֶל־יוֹסֵ֖ף לֵאמֹ֑ר אָבִ֣יךָ צִוָּ֔ה לִפְנֵ֥י מוֹת֖וֹ לֵאמֹֽר׃ (יז) כֹּֽה־תֹאמְר֣וּ לְיוֹסֵ֗ף אָ֣נָּ֡א שָׂ֣א נָ֠א פֶּ֣שַׁע אַחֶ֤יךָ וְחַטָּאתָם֙ כִּי־רָעָ֣ה גְמָל֔וּךָ וְעַתָּה֙ שָׂ֣א נָ֔א לְפֶ֥שַׁע עַבְדֵ֖י אֱלֹהֵ֣י אָבִ֑יךָ וַיֵּ֥בְךְּ יוֹסֵ֖ף בְּדַבְּרָ֥ם אֵלָֽיו׃
(16) So they sent this message to Joseph, “Before his death your father left this instruction: (17) So shall you say to Joseph, ‘Forgive, I urge you, the offense and guilt of your brothers who treated you so harshly.’ Therefore, please forgive the offense of the servants of the God of your father’s [house].” And Joseph was in tears as they spoke to him.
אביך צוה. שִׁנּוּ בַדָּבָר מִפְּנֵי הַשָּׁלוֹם, כִּי לֹא צִוָּה יַעֲקֹב כֵּן, שֶׁלֹּא נֶחֱשַׁד יוֹסֵף בְּעֵינָיו (בראשית רבה, יבמות ס"ה):
אביך צוה THY FATHER DID COMMAND — They altered the facts (they stated something that was false) for the sake of peace, for Jacob had given them no such command because Joseph was not suspect in his sight (Yevamot 65b)).
(בראשית נ, טז): וַיְצַוּוּ אֶל יוֹסֵף לֵאמֹר אָבִיךָ צִוָּה וגו', תָּנֵי רַבִּי שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר גָּדוֹל הַשָּׁלוֹם שֶׁאַף הַשְּׁבָטִים דִּבְּרוּ דְבָרִים בְּדוּיִם בִּשְׁבִיל לְהַטִּיל שָׁלוֹם בֵּין יוֹסֵף לַשְּׁבָטִים, הֲדָא הוּא דִכְתִיב: וַיְצַוּוּ אֶל יוֹסֵף לֵאמֹר וגו', וְהֵיכָן צִוָּה לֹא מָצִינוּ שֶׁצִּוָּה.
“They instructed to tell Joseph, saying: Your father instructed before his death, saying, so say to Joseph: Please, forgive the transgression of your brothers and their sin as they did evil to you. And now, please forgive the transgression of the servants of the God of your father. Joseph wept as they spoke to him” (Genesis 50:16–17). “They instructed to tell Joseph, saying: Your father instructed…” It is taught: Rabbi Shimon ben Gamliel says: Great is peace, as even the tribes spoke fabricated matters in order to impose peace between Joseph and the tribes. That is what is written: “They instructed to tell Joseph, saying…” Where did he instruct? We do not find that he instructed. “So say to Joseph…[Joseph wept as they spoke to him]” – he said: ‘This is how my brothers are suspicious of me?’
Genesis Rabba seems to be implying that Joseph wept because he saw through his brother's "white lie" as as way to protect themselves from the consequences of their actions against Joseph. Compare this to the Torah's classic white lie, in which God lied to spare Abraham's feelings.
(יב) וַתִּצְחַ֥ק שָׂרָ֖ה בְּקִרְבָּ֣הּ לֵאמֹ֑ר אַחֲרֵ֤י בְלֹתִי֙ הָֽיְתָה־לִּ֣י עֶדְנָ֔ה וַֽאדֹנִ֖י זָקֵֽן׃ (יג) וַיֹּ֥אמֶר יְהֹוָ֖ה אֶל־אַבְרָהָ֑ם לָ֣מָּה זֶּה֩ צָחֲקָ֨ה שָׂרָ֜ה לֵאמֹ֗ר הַאַ֥ף אֻמְנָ֛ם אֵלֵ֖ד וַאֲנִ֥י זָקַֽנְתִּי׃
(12) And Sarah laughed to herself, saying, “Now that I’ve lost the ability, am I to have enjoyment—with my husband so old?” (13) Then יהוה said to Abraham, “Why did Sarah laugh, saying, ‘Shall I in truth bear a child, old as I am?’