Avrohom ordered Sarah to say that she is his sister (he was afraid that Egyptian kill him). How was Esther not afraid that Achashverosh kills Mordechai? (Esther 8, 1[1])
She was the wife of Mordechai.
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The Gemara (Megila 13A) says.
A Tanna taught in the name of R`Meir: Read not 'for a daughter' [le-bath], but 'for a house' [le-bayith].(Ester 2, 7) Similarly it says: But the poor man had nothing save one little ewe lamb, which he had brought up and reared; and it grew up together with him, and with his children; it did eat of his own morsel, and drank of his own cup, and lay in his bosom, and was unto him as a daughter.
[1] On that day did the king Ahasuerus give the house of Haman the Jews' enemy unto Esther the queen. And Mordecai came before the king; for Esther had told what he was unto her.
א בַּיּוֹם הַהוּא, נָתַן הַמֶּלֶךְ אֲחַשְׁוֵרוֹשׁ לְאֶסְתֵּר הַמַּלְכָּה, אֶת-בֵּית הָמָן, צֹרֵר היהודיים (הַיְּהוּדִים); וּמָרְדֳּכַי, בָּא לִפְנֵי הַמֶּלֶךְ--כִּי-הִגִּידָה אֶסְתֵּר, מַה הוּא-לָהּ.