Hi, I also had problems with this achievement and I've tried three different ways:
1) normal game, standard size map, prince difficulty - all capitals have the same religion, including the city states.
No achievement, but I captured 2 capitals before they assume my religion.
2) Hot seat game, standard size map, settler difficulty, all 8 players controlled by me.
5 different religions (I think thats the max number with this settings).
Also no achievement, after all capitals assume the same religion...
3) Same settings as suggest by
Haphaz (except the map type):
Hotseat, Prince, 2 player (both me!), random civs, Standard map, CONTINENTS, NO CITY STATES.
Player 1 (Arabia) explored a religion (Buddhism, establish in rd 84)
Player 2 (Persia) has only a pantheon
After round 91, the achievement popped for no seeming reason... No idea why...
I've attached my savegames: rd 91, the rd before I get the achievement ans rd 92, the magic round for "we are family"
Maybe someone wants to look at my game and can explain WHY I get in in that special round. (path for the savegames is ...\My Games\Sid Meier's Civilization 5\Saves\hotseat\auto\)
so long, MrOz80
Sorry for spelling mistakes or wrong "Civilization terms", english is not my native language