Mother doesn't know me. Any advice?

I agree. I wouldn't hold any expectations of her remembering you or anything else. As she progresses, there will likely be loss of much more, including mobility, eating, etc. Whatever she says should be golden. Agree and provide comfort. Doing otherwise is really not productive and may only agitate, confuse or frighten her. I try to take the lead from my LO. She can't really talk anymore, but, when she did and she asked about people, I tried to see how much she recalled and if she asked about someone, I'd say that they certainly loved her and if they could they would come soon or, I'd say that they had called and left her a message that they loved her. I didn't even know who she was talking about, but, it didn't matter.  This seemed to calm her and assure her she wasn't forgotten.

 It's painful to be forgotten as the primary caregiver, as that has recently happened to me, but, I'm trying to just give positive thoughts and love. Certainly, she can feel that, even if she doesn't know I'm her cousin.

You have already discovered that telling her YOU are her daughter doesn't help so why would you persist in doing that? People with dementia live in an alternate reality, perhaps in her mind she is a young mother and the daughter she is looking for is a little girl. Even if she can't see you for who you are isn't it much better for both of you if she sees you as a pleasant friend rather than someone who is challenging her perceptions?

edit: there are some wonderful videos by Teepa Snow on YouTube that can help you understand this part of dementia