Besides Yenna's plot involvement in the camp, is there supposed to be a questline for finding her mother? I never really agreed to help her so maybe that's why I don't have anything related to that (and if that's the case how could I initiate such quest afterwards?)
No, you cannot find Yenna's mother. Nor is there a quest to attempt to do so.
The presumed "why" is actually pretty interesting, and it's entirely because of the particular role Yenna plays in the overall plot -- she's there as a schrödinger's trojan horse.
Act 3 Spoilers
During Act 3, Orin, a Changeling, steals one of your party members and holds them captive. There are three criteria:
- The captured party member can't be a member of the active party;
- The captured party member must be Lae'zel, Halsin|Minthara, or Gale;
- The captured party member can't be romanced by the player
These requirements are actually pretty strict - it's easy to deny Orin a victim. In such scenarios, Yenna is the fallback, with Orin capturing her instead (and murdering poor Grub the cat).
This does lead to some strangeness though -- Yenna disappears from your camp once Orin's machinations are revealed, even if a valid companion was found. There's no follow-up in that case; Yenna is simply gone.
In short, Yenna is the backup resolution to ensure the game plot progresses normally. The fact that her mother is missing simply gives her the plausible deniability to insert herself innocuously into your camp.