Mother tell MIB there is nothing across the sea, Jacob doesn't know how to lie

At the caves, Mother is working on a tapestry as Jacob returns (we see where he got his knack for tapestries). She asks him where his brother is and he tells her he is at the beach, staring out at the ocean. Mother looks at Jacob suspiciously. Jacob asks if he can help her and she tells him to sort some thread.

Mother asks what Jacob and MIB were doing at the beach. Jacob hesitates, and then says they were just walking. She turns to him and asks "Do you love me Jacob?" He says yes and then she asks him to tell her what happened.

We then see MIB sitting at the beach, looking at a sea turtle. Mother approaches, sits down with him and smiles. MIB realizes that Jacob told her about the Senet game he found. Mother says that Jacob does not know how to lie because he is not like MIB. He asks what is he like and she tells him that he's special.

He then asks if he can keep the game and she says of course and that she left it there for him. Presumably, this is a lie because she doesn't want him to know that there is a world beyond the island. She asks where else could it have come from, and he says from somewhere else, across the sea. Mother says that there is nowhere else and the island is all there is.

MIB asks where did they come from then, and Mother says that he and Jacob came from her and she came from her mother. He asks where her mother is and she says that her mother is dead. MIB asks what dead is and she tells him that it's something he'll never have to worry about.

So clearly, we see that Mother certainly has more in common with MIB than Jacob. She is manipulative and certainly knows how to lie, but is she truly crazy like Locke said earlier this season? I guess it's subjective.