The mom and auntie Ren definitely come across as exes, it seems like they on relatively good terms though which is nice.
I think that should be the case, yeah. Taking the elements we have and filling in the gaps with the simplest possible explanations in a Ockham's razor-y kind of way, I end up with something like this:
1) Mom and auntie Ren were lovers in college.
2) Mom dumped auntie Ren because, y'know, the usual reasons: "Gotta stop playing around, gotta grow up, gotta do my filial duty, gotta marry and have children!"
3) Mom married some guy and had Youzi. Then she divorced him because she's a lesbian and having to put out for him sucked big time and she couldn't take it anymore.
4) Mom became a famous actress, Ren became a famous singer.
5) They meet again (partly thanks to Youzi) and Ren finds out that mom is living alone and raising her kid by herself... and that she often sings to Youzi the secret love song she and Ren wrote together while in college.
Present time, Youzi suspects that mom and auntie Ren still love each other but are silly & useless about it. Maybe mom also feels guilty for hurting Ren before and feels she doesn't deserve to get her back.
This are my best guesses given the evidence we have.