What are the requirements to unlock "We are family"?

Question

We are Family

Become the dominant Religion in every Capital city on a standard-size or larger map.

I have set up a game on a huge map and my religion was spread over all cities, while 35 of them belonged to me. So my religion was clearly dominant. I also made sure they are dominant in the enemies capital cities.

But the achievement did still not unlock. While the description seems to be very obvious, a lot of people seem to have a problem with it. I found a couple of sites with discussions about it, including civfanatics, but no clear answer on how to actually unlock it.

Answer 1

I covered this in the G&K section of my multi-part guide to achievements in Civ5. It is definitely glitchy, but if you're willing to cheese it in a hotseat game, you can just keep trying until it unlocks:

I was able to get it in a quick ancient-era hotseat game, on a Standard size map, with 2 civilizations and no city states. After establishing religions in both capitals, I sent Great Prophets across to each opposing capital. The first time I converted the opposing capital to my religion, it didn't take. Later, the pressure from the holy city flipped the religion back, and I tried again. On the second try, it unlocked. There's definitely something glitchy here, but hotseat games do work for it. You may have to attempt this achievement multiple times.

There's no need to finish the game afterwards, the unlock occurs pretty quickly after the event. I don't believe I had any other cities except for the capital.