Csdrhrt

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 there were from and why the Torah does 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.