אבן העזר ט״וEven HaEzer 15

Se’if 1א׳

אֵלּוּ שֶׁאֲסוּרוֹת מֵחֲמַת עֶרְוָה, מֵהֶן מִן הַתּוֹרָה מֵהֶן מִדְּרַבָּנָן. אוֹתָן שֶׁהֵן מִן הַתּוֹרָה, אֵין קִדּוּשִׁין תּוֹפְסִין בָּהֶן; וְאוֹתָן שֶׁהֵם מִדְּרַבָּנָן, קִדּוּשִׁין תּוֹפְסִין בָּהֶם וּצְרִיכוֹת גֵּט. וְכֵן הַמְקַדֵּשׁ סְפֵק עֶרְוָה צְרִיכָה גֵּט:

These are the women forbidden on account of incest, whether prohibited by our Torah or by our sages. With regard to those forbidden by the Torah, [if one attempts a wedding] the marriage does not take effect. With regard to those forbidden by our rabbis, the marriage takes effect and requires a get (Jewish divorce). And thus if one betroths a woman about whom there is "suspicion of incest" a divorce is required.

Se’if 2ב׳

אִמּוֹ, אֲסוּרָה לוֹ מִן הַתּוֹרָה. אֲבָל אֵם אִמּוֹ אֵינוֹ אֲסוּרָה לוֹ אֶלָּא מִדְּרַבָּנָן, וְזוֹ אֵין לָהּ הֶפְסֵק, אֲפִלּוּ אֵם אֵם אֵם אִמּוֹ עַד מַעְלָה מַעְלָה, אֲסוּרָה:

Marrying one's mother is forbidden by the Torah. However, the mother of one's mother is only forbidden by our sages, and there is no limit to this prohibition; even the mother's mother's mother of his mother, etc. is forbidden.

Se’if 3ג׳

אֵם אֲבִי אִמּוֹ אֲסוּרָה לוֹ מִדְּרַבָּנָן, הִיא בִּלְבַד:

The mother of his mother's father is forbidden to him according to the sages, and she alone.