Mom with Alzheimer's sometimes thinks she has another daughter who does things for/with her when it's actually me. Need advice.

She seems to recognize me (most of the time) when I'm with her, but then she calls to tell me things this "other girl" is doing. She implies in the phone calls that she doesn't completely trust this other girl who is "acting like her daughter". I've been concerned, but expected behavior like this as her disease progresses.

What scares me is that she recently said this new person coming around is a daughter she gave up for adoption when she was 19. I just found out she did in fact give up a child at 19, 6 years before I was born. My dad doesn't know about this other child, so he thinks she's just confused about me, but I now understand the problem on a different level. Obviously, this is not something we have ever discussed in my family.

I'm not sure how to handle this. I feel like I need to talk to the doctor about her having delusions, but I'm not sure what to do about the "other daughter" situation. I can't imagine telling my dad that she gave up a child years ago (not his) - I don't feel like that's my place, but I'm scared I'm hurting everyone even more by not telling him.

Thanks for any help you can provide.