Here is a character whose arc on the show is about embedding her in a functional group, making her feel special by having her be a part of something special. What could speak to this notion more than family? This character is technically adopted, and an only child – she is, on paper, lacking the comfort and solace of a birth-given sense of belonging. So the show introduces her real mom, and establishes that this character, the daughter, needs a mother in her life. Here’s the girl’s chance to feel like she belongs – regardless of however much she loves her dads, the show specifically set up the idea that Rachel longs for a mother – for that sense of fitting in, of knowing where she comes from and where she belongs.
And after four episodes, Shelby basically says she doesn’t really want a relationship with Rachel, but would rather have a new baby to start fresh with. And she leaves. Oh goodness, the heartache. The heartache! This is in direct violation of Rachel’s character development! Rachel is being deprived of a family, again! Rachel is being told she doesn’t belong, again! And that’s not to say that it’s not a worthwhile storyline to explore, but oh goodness, does it hurt. Rachel’s mother actively denying her a relationship falls in step with the notion that Rachel is doomed to isolation in all areas. She is awash in a sea of functional relationships, repeatedly refused healthy interactions, friendships, and romances.
– Rachel Berry in the Spotlight: Rachel, Shelby, and the Concept of Family