Roger
IIRC there are a number of different circumstances in which cannibalism took place in PNG. The specific one that you are referring to took place amongst a people called the Fore where, yes, it was a ritual thing and took place within kinship groups. the problem was that as the disease, like CJD, may stay dormant for decades before becoming symptomatic it was very difficult for the Fore to realise the link between the practice and the disease.
It is worth noting that although the practice had ceased by 1960, the last incidence of the disease, kuru, as they called it, emerged as recently as 2006.
Scary!