Мусар к Берешит 18:18

This message is also alluded to in the wording of Genesis 1,3 where G–d gave the directive יהי אור, and the Torah reports: ויהי אור. The letters in G–d's name י-ה-ו-ה, are really only י-ה-ו, seeing that the letter ה appears twice. If Adam had not sinned, the name would have been י-ה-ו-ה. Unfortunately, the spiritual level necessary for that name never existed, not even when we merited the building of the Holy Temple, at which time the Ineffable Name of G–d was of the י-ה-ו-ה variety. This is what Solomon refers to in Kohelet 11,3 that במקום שיפול העץ שם י-ה-ו, the word שם has to be vocalised Shem, i.e. the Name will consist of the letters Yud-Heh-Vav. Genesis 1,3 hints at G–d's original plan which provided for יהי instead of יהו. When the Torah reports ויהי אור, i.e featuring the three letters of the present "full" name of G–d, this is a hint that part of the "great light" was already withheld; G–d anticipated Adam's נפילה, "fall." אדם הראשון caused that "fall"' by eating from the tree of knowledge. The meaning of Kohelet is now clear. "The place the tree fell is the cause that the letters in the name of G–d remain י-ה-ו. The world as it is cannot rise above that level of G–d's name. When G–d says of Abraham in Genesis 18, 18 that ואברהם היו יהיה לגוי גדול, we have the implied promise that in the future, as a result of someone like Abraham, that configuration described as “היו,” will eventually be changed to the configuration “יהיה.” If Abraham is previously described as standing under the "tree,” this is a hint that he "stands guard,” insures that the original design for the world will materialise: that the fall from grace due to having eaten from the tree will be reversed.

I have explained earlier how Abraham is the source of the fulfilment of G–d's Name becoming י-ה-י-ה instead of י-ה-ו-ה, and how this is alluded to in the name Abraham called Mount Moriah, i.e. השם יראה.

Having sexual intercourse with two sisters [while they are both alive, Ed.] is perceived as equivalent to damaging the equilibrium in the domain הוד ומלכות. Only our patriarch Jacob, whom G–d Himself had called "אל" a celestial being (Genesis 33,20, cf. Megillah 18a), attained a level of spiritual perfection that permitted him to use the sceptre normally only used by the "King" in the Celestial Regions. In connection with the prohibition to marry two sisters simultaneously, the Torah (18,18) uses the expression לצרור לגלות ערותה, which we may provisionally translate as "as a rival." Rabbi Abraham Saba, in his Tzror Hamor understands the word as "creating a knot," i.e. tying up Celestial conduits and preventing them from exerting their beneficial influence on their respective partners in the domains of the emanations, and through them on this world. [This comment of the Tzror Hamor is not found in his commentary on this chapter in our פרשה. Ed.]