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I just met an Israeli woman, who said that she became a widow when her first and only child was five years old.

She said that if it wasn't for her husband having more than one wife, with whom she got along fine and co-raised her child together, she would have ended up in a bad situation. That is all the information I received from her.

My question has nothing to do with whether polygamy is right or wrong, and therefore one can refrain from such answers.

My question is: Is there anything in the Tanakh or Talmud about sister-wives (or better co-wives?) helping raise the child of other sister-wives (or co-wives) upon widowhood (or absence of husband)?

I was not going to ask my question, but I was inspired by this:
Is there anything in the Tanakh or Talmud about a woman that raises her children alone?

Note: This is an example of co-wives helping each other raise their children with complete lack of presence of the father (Yaakov) who was working like a slave day and night see bereishit 31,40 (for Lavan)and was effectively not able to be present to raise his kids .
When Rachel's maidservant Bilhah gave birth to Dan, Rachel said Bereishit 30,6: "G-d judged me and listened to me and gave me a son" which means she raised Dan as a son even though she was not his biological mother (no doubt Bilhah also played a major role raising Dan as well).

Another example is after Rachel died of childbirth and Yaakov became a widower from his main wife, Bilhah in turn raised Rachel's children. Rashi quoting Midrash Rabba 84,11 asks regarding Yosefs dream of the sun moon and stars bowing down to him Bereishis 37,10:

"הבוא נבוא אני ואמך"
- והלא אמך כבר מתה? והוא לא היה יודע שהדברים מגיעים לבלהה שגדלתו כאמו

-if Rochel was dead who was the moon? The moon refers to Bilhah who raised Yosef like a mother