I'm not sure I understand what the Nanny was doing? If she was trying to earn the MC's trust, and make the MC only believe in her, why hide her away from the doctor when she's sick? How does doing that help in her mission?... Unless she was actually poisoning the MC, but since the doctor didn't see anything wrong with her, I guess that's not the case?
Really, the whole plot is a bit nonsensical. The MC often wonder if her family loves her, and she feels like she shouldn't get close to them, but she spent most of her past life getting revenge for them, because they had saved her, and shown their love of MC. It would make more sense if she was trying to understand WHY she was treated like she was, rather than thinking she wasn't/isn't loved - especially when they're visiting her at night and showing her affection then.