Nepenthes and Cephalotus are both in their own families. I'm not sure how they're related above that classification. Sarracenia, Heliamphora and Darlingtonia are all in the same family. I could be wrong, but I think Heliamphora is supposedly the most primitive of the 3. As far as evolving to occupy the same niche, I'm not sure the 3 families occupy the same niche. Yes, they all evolved carnivory, but I think the plants in the Cephalotus family,Nepenthes family, and Sarracenia family occupy different niches. I think the carnivory is an afterthought, so to speak.