Inauthenticity comes from a lack of awareness. An inauthentic person is someone whose choices and actions do not correspond to his or her own self. Another way to say it is an inauthentic person is stuck in his or her ego. Inauthenticity is thus a state and lying and being sincere are actions which emerge from that state.
We lie when we intentionally intend to hide something from others or from ourselves whether for malicious or benevolent purposes. Lying dispossesses the person who is lied to of his or her free will.
Sincerity is an action that derives from being located in the context of our ego while thinking we are telling the truth. Sincerity gives the appearance of telling the truth while one is actually in a state of inauthenticity.
The only access to freedom is authenticity: being true to one Self.