(א) וזכר שם נשותיהם לכבוד אברם, וכי שנה האל שם שרי לשרה, וממלכה גם כן היתה רבקה שהיתה אשת יצחק.
(1) They are specifically mentioned because God would change Sarai's name to Sarah, and because Milcah was the grandmother of Isaac's wife, Rebekah.
The Torah: A Women's Commentary on Genesis 11:29
Nahor's wife was named Milcah. Her name, like Sarai's, evokes royalty (malcah means "queen").
Daughter of Haran, father of Milcah. The double identity markers for Milcah draw attention to her unique position in the ancestral lineage. It also discloses that she married her uncle. Rebekah, the second matriarch, will identify herself as Milcah's granddaughter(24:24), in the Bible's sole unambiguous mention of someone's grandmother. This reference highlights her importance.
The family tree:
https://en.wikipedia.org/wiki/Haran
The Torah: A Women's Commentary on Genesis 22:20
Milcah. We were told of her marriage to Nahor when she was first introduced with Sarai (11:29), before Abram's household left the city of Haran for Canaan. Now she is mentioned again--and once more in verse 23--just before Sarah dies (23:1-2). Rebekah will also mention her grandmother Milcah when she identifies herself (24:24, 47). It is not clear why Milcah merits her relative prominence.
(א) אשר ילדה לבתואל בן מלכה בעבור שהיה לנחור בנים מפלגשו ראומה ייחסו לעולם בתואל בן מלכה ובעבור שהנערה הזכירה אם אביה תחלה בת בתואל אנכי בן מלכה כי כן דרך הנערות כדרך ותגד לבית אמה (להלן פסוק כח) לכן יזכירנו הכתוב כי הוא בן מלכה אשת נחור אבל העבד אמר ותאמר בת בתואל בן נחור (שם מז) כי תיקן הדבר בדרך מוסר אבל אמר אשר ילדה לו מלכה לומר שהוא בן הגבירה:
(1) She who had been born to Bethuel the son of Milcah. Because Nahor had children from his concubine Reumah, Scripture always mentions Bethuel's parentage as Bethuel, the son of Milcah, [to stress that he was born to Nahor from his wife (Milcah), and not from his concubine (Reumah).
And because the maiden mentioned her father's mother first, I am the daughter of Bethuel the son of Milcah (v. 24) for such is the manner of young maidens [to have a closer relationship with their mother than their faither], as She told her mother's household (v. 28), therefore Scripture mentions that he was the son of Milcah, the wife of Nahor, in that order. The servant, however, said, And she said, "The daughter of Bethuel, the son of Nahor, whom Milcah bore to him (v. 47), for he corrected the matter out of propriety. But he said whom Milcah bore to him, to indicate that he was the son of the wife.
Rabbi Samson Raphael Hirsch on Genesis 24:15
Here, as also in Rivkah's reply, her lineage is traced back to her grandmother, but Rivkah's mother is not mentioned. Apparently, the mother was not of Avraham's family, nor was she imbued with his spirit. It seems that Rivkah took after her grandmother, whereas her brother took after the mother.
Rabbi Samson Raphael Hirsch on Genesis 24:24
She emphasizes the names of Milcah and Nachor, who as sister and brother of Sarah and Avraham, were likely to be known.
Ellen Frankel, The Five Books of Miriam, p. 33
OUR DAUGHTERS ASK: Who is Milcah? As it is written, [Rebecca] was born to Bethuel ,the son of Milcah, the wife of Abraham's brother Nahor (24:15). How unusual for the Torah to include Rebecca's grandmother in her pedigree.
WILY REBECCA ANSWERS: Milcah was a central figure in my family--sister to Lot, sister-in-law and niece to Abraham; my grandmother. A link to three generations. But how little I know about her life! Besides her place in the family tree, all I know is that her name means "queen." But what a profound influence she had!
MILCAH ADDS: It's easy to understand why I was such a veiled figure. I was overshadowed by two headstrong men--my uncle Abraham and my uncle Nahor, who later became my husband. Uprooted from my home and friends, transplanted to a desolate waste far from the grand city of my youth, I retreated into my tent and found solace in my children and grandchildren, especially Rebecca, the apple of my eye. I never doubted that she would take her future into her own hands.