Narcissa is as cold as Skye is warm. Both beautiful in their own way, of course, but Lily never knew beauty could vary so widely.
When she kissed Narcissa, it was like holding an ice statue. Pale and lovely, but ultimately something that would melt away if there was too much warmth between them. Narcissa was never hers, not really, and Lily knows it was inevitable that she would lose her to another cold thing like Lucius, in the end.
When she kisses Skye, she feels all her warmth returned. And perhaps she will lose her to Peter, one day, but Lily would never begrudge them that. Because until that day comes, she knows Skye is all hers.