So my MC's sister has just been killed by evil forces, leading her into a dark search for the reason. This has so far lead my MC into an occult bookstore/dance club (Yeah, I know) Wherein she meets a woman who knew her sister, who then decides to help my MC on her quest.

Why is she helping my MC, whom she barely knows, on a quest that has some SERIOUS potential for danger, for a dead woman whom she also didn't know very well?

Halp.

Because she knows who was responsible for the sister's death, or suspects she does, and she has personal reasons of her own to want to oppose that power. For instance, it killed someone dear to her, or it destroyed something that was important to her.

Or, she could just feel something for your protagonist. Maybe he looks strikingly like someone she knew who was destroyed by this being and who could have survived if she'd helped that person in the way your main character wants to be helped.

Excellent! Thank you!