parshanut torah comment - Why did Rivka consult Shem, and not Avraham or Yitzchok?

The Posuk says (Gen. 25:22) regarding Rivka:

וַיִּתְרֹצֲצוּ הַבָּנִים בְּקִרְבָּהּ וַתֹּאמֶר אִם כֵּן לָמָּה זֶּה אָנֹכִי וַתֵּלֶךְ לִדְרֹשׁ אֶת יְ־הֹוָ־ה

Translated as: And the children struggled within her, and she said, "If [it be] so, why am I [like] this?" And she went to inquire of the Lord.

Rashi comments on this:

ותלך לדרוש: לבית מדרשו של שם

Translated as: And she went to inquire - to the academy of Shem.

The question is why go to Shem and not Avraham or Yitzchok, and what happened to Eiver?