Great Mothers' Leader Ability, Threads of Destiny reads as follows in pertinent part (it's really long): If all allies are Nightsister at the start of battle, at the start of each encounter and end...
Ideas
I don't know about the first part (someone taking a turn between the two Fates), but I experienced the second part (Doomed enemy dies, GMs using their two regular abilities instead of Fate Weaving) a few hours ago today, in Conquest I think. Either in Conquest or in S&S.