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While the forefathers were very cautious in choosing their spouses from Avraham's family, the Torah says nothing about Bilhah and Zilpah's ancestry.
What nation/family were they from and why does the Torah not see the importance of telling us about that?
According to Bereishit Rabbah 74:13 Bilhah and Zilpah were also daughters of Lavan.
וַיַּעַן לָבָן וַיֹּאמֶר לְיַעֲקֹב הַבָּנוֹת בְּנֹתַי וְהַבָּנִים בָּנַי (בראשית לא, מג), אָמַר רַבִּי אָבִין כּוּלְּהוֹן בְּנוֹתָיו הָיוּ, הַבָּנוֹת בְּנֹתַי, הֲרֵי שְׁתַּיִם. (בראשית לא, מג): וְלִבְנֹתַי מָה אֶעֱשֶׂה, הֲרֵי אַרְבַּע. רַבָּנָן מַיְתֵי לָהּ מֵהָכָא (בראשית לא, נ): אִם תְּעַנֶּה אֶת בְּנֹתַי, הֲרֵי שְׁתַּיִם, (בראשית לא, נ): וְאִם תִּקַּח נָשִׁים עַל בְּנֹתַי, הֲרֵי אַרְבַּע.
"And Lavan spoke up and said to Ya'akov, 'The girls are my daughters, and the boys my sons.'" (Bereishit 31:43)
Said R. Avin, 'They were all his daughters: "The girls are my daughters" gives two, "And as for my daughters, what should I do?" (Bereishit 31:43) gives [another two equaling] four.'
The Rabbis brought it from here: "If you afflict my daughters" (Bereishit 31:50) gives two. "And if you take other wives in addition to my daughters" gives [another two equaling] four.
Other than the view brought by @JoelK, there's also the view brought in 2-3 sources:
The apocryphal Testament of Naftali, part of the Testament of the Twelve Patriarchs. According to the Greek version, Bilhah and Zilpah were daughters of Rotheus, who was a member of Avraham's household as well as the brother of Devorah, Rivka's nurse. Their mother was named Ednan/Eve/Hannah (depending on the particular manuscript). Rotheus fell captive and was bought by Lavan, who eventually married him off to one of his slaves, the aforementioned Ednan/Eve/Hannah.
A fragment of the Hebrew Testament of Naftali was found at Qumran, 4Q215. Per that version, their parents were called Achiyot and Channah. Achiyot was freed from captivity by Lavan and given a wife.
Lastly, in Rabbi Moshe Hadarshan's Beresheet Rabbati, p. 119 (Albeck edition), the same tradition is brought. In this version, their parents' names were Achoti and Chavah/Channah (unclear in the manuscript). Achoti was freed from captivity by Lavan and given a wife.
For more information on these three sources, see M. E. Stone, 'The Genealogy of Bilhah', Dead Sea Discoveries 3 (1996), pp. 20-36.
As for why we don't know these things from pshat, that's a very good question. I don't have an answer for that, but the question is relevant for many things in Tanach, such as Sarah's lineage (her identification with Yiskah, daughter of Haran isn't pshat) or the name of David's mother (it's the gemara who tells us that her name was Nitzvat bat Adel).
