ותשקין וגו'. יַיִן נִזְדַּמֵּן לָהֶן בַּמְּעָרָה לְהוֹצִיא מֵהֶן שְׁנֵי אֻמּוֹת: 'ותשקין וגו AND THEY MADE THEIR FATHER DRINK [WINE] etc. — Wine was at hand for them in the cave out of a set purpose that they might bring forth two nations (Midrash Tanchuma, Beshalach 12).
ותשכב את אביה. וּבַצְּעִירָה כְתִיב וַתִּשְׁכַּב עִמּוֹ? צְעִירָה, לְפִי שֶׁלֹּא פָּתְחָה בִזְנוּת אֶלָּא אֲחוֹתָהּ לִמְּדַתָּה חָסַךְ עָלֶיהָ הַכָּתוּב וְלֹא פֵּרֵשׁ גְּנוּתָהּ; אֲבָל בְּכִירָה שֶׁפָּתְחָה בִזְנוּת פִּרְסְמָהּ הַכָּתוּב בִּמְפֹרָשׁ. ותשכב את אביה AND SHE LAY WITH HER FATHER — In the case of the younger daughter it is written (Genesis 19:35) “and she lay with him”, and it does not state “she lay with her father”. But because the younger did not originate this unchaste conduct but her sister taught it to her, Scripture glosses over her sin and does not explicitly make mention of her shame; but since the elder originated this unchaste conduct Scripture exposes her fully (Numbers Rabbah 20:23).
ובקומה. שֶׁל בְּכִירָה נָקוּד, לוֹמַר, שֶׁבְּקוּמָהּ יָדַע, וְאַעַפּ"כֵ לֹא נִשְׁמַר לַיִל שֵׁנִי מִלִּשְׁתּוֹת. אָמַר רַבִּי לֵוִי כָּל מִי שֶׁהוּא לָהוּט אַחַר בּוּלְמוּס שֶׁל עֲרָיוֹת לַסּוֹף מַאֲכִילִים אוֹתוֹ מִבְּשָׂרוֹ: ובקומה NOR WHEN SHE AROSE — This word where it occurs with reference to the elder sister (Genesis 19:33) has dots above it (as though it is not written at all), implying that when she arose, he (Lot) was aware of it, and yet he did not take care on the second night to abstain from wine. (Horayot 10b.) R. Levi said, Whoever is inflamed by sexual desire will, in the end, be made to eat his own flesh (Genesis Rabbah 51:9).