The Six Mothers
The Midrash counts Bilhah and Zilpah among the mothers of Israel, alongside Sarah, Rivkah, Rachel and Leah.
When the Mishkan was set up, the princes of the tribes brought their offering before Hashem: six covered wagons and twelve oxen. The Midrash gives several meanings for the number six, and one of them is that the six wagons stood for the six mothers of Israel: Sarah and Rivkah, Rachel and Leah, Bilhah and Zilpah. The same teaching appears in other places in the Midrash Rabbah. Zilpah came into Yaakov's house as a maid and was given to him as a wife by her mistress, but this Midrash places her among the Imahos. We may learn from it that every tribe had a mother, and that every one of those mothers is remembered when the whole people of Israel stands together before Hashem.
Source: Bamidbar Rabbah 12:17 (on Bamidbar 7:3); also Esther Rabbah 1:12 and Shir HaShirim Rabbah 6:4
The Midrash made sure Zilpah was counted when Israel brought its gifts to the Mishkan. At your next family simchah or Shabbos meal, think about who usually goes unmentioned: a step-parent, a caregiver, an in-law, a teacher who helped raise your children. Name them in your words of thanks, or ask one of them to say a few words. It takes a moment, and it lets everyone at the table know that they are part of the family's story.