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)
She was the wife of Mordechai.
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.
 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.