Commentary for Sanhedrin 116:9

אמר רב הונא כותי מותר בבתו וא"ת מפני מה לא נשא אדם את בתו כדי שישא קין את אחותו משום עולם חסד יבנה

So that Cain should marry his sister, as it is written, For I said, the world shall be built up by grace.<span class="x" onmousemove="('comment',' Ps. LXXXIX, 2. It was an act of grace on Adam's part to deny himself his sister; or, as Rashi states, God commanded Adam to deal graciously with Cain, so that Cain, by marrying her, should build up the world. ');">¹²</span>

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