"A sister! you are she," Olivia had said, when truth had come to light and Cesario changed to Viola, and had meant it with all her heart. Everything had sorted itself out in that moment, and if Sebastian isn't Cesario, he also isn't a girl or Duke Orsino. So Olivia is almost grateful to Viola, despite the deception.
Sebastian isn't Cesario, but he is witty and generous, caring in a way she hasn't experienced since the death of her brother (and that thought doesn't hurt as it would have, six months ago). He has managed to make a place for himself without taking hers away; has managed even to make the fool laugh, which is a sight she never thought to see.
And when she wakes in the middle of the night, half-sick from dreams where Viola is Cesario is Sebastian mocking her for not being able to tell woman from man, he drapes an arm across her belly and rests his forehead against her cheek. Whispers sleep in her ear until she can close her eyes again. Looks at her in the morning sunlight and smiles as if he might love her.