The Two Sisters are not so much opposites as complements to each other. It is interesting that Isis and Nephthys seem to have become attached to different aspects of Hathor in Their association with Her. Sad at Heart Nephthys became connected with Hathor, Lady of Joy and Divine Intoxication. Lady of Governance Isis became connected with Hathor the soft-eyed Cow Mother, the Mother of the God, and the Lady of Amentet. Yet, as always, these roles are fluid and the Two Sisters flow into one another, even as They express different aspects of Their Divinity.
Isis & Her Dark Twin, Nephthys
Glyn Hnutu-healh: History, Alchemy, and Me 