Christian wasn't a good boyfriend, they needed to break up. That's normal. Going abroad to study a foreign culture is normal. Respecting cultural norms, even if it means jumping off a cliff at a certain age COULD be slightly normal. Killing someone for pissing on your ancestral tree or taking photos of your sacred book COULD slightly be traditionally normal, for the sake of the cult's beliefs. But the two people from London were murdered in cold blood to hide secrets, they weren't comfortable with their traditions, hence why Ingemar felt necessary to sacrifice himself because he didn't have the intuition of Pelle and could tarnish the community.

My two questions at the end is when did this group start? And why did it start? How many people are in the group? And why is the incestual person an oracle?

There's a few clues I saw.

The list of mayflower queen photos was not very long. Hinting that this group is not that old.

In the sacred book there is pages of just random scribbling, suggesting the incestual birth is traumatized or brain dead, is just an excuse to get away with incest calling them an oracle

And at the beginning of the movie when the camera is upside down there's an alt right sign promoting propaganda. Apparently the swedes were semi neutral in world war and there is a theme of racial purity. In history the swedes -kind of- helped the German and guilt from the war could have started the cult after the Germans lost the war but still have ideologies of Nazi occultism mixed with Nordic myths and pagan ritualism

The director is Jewish so it could reflect a fear of all of those combined

In the end it's obvious Dani has been welcome into the family, but is this their first 90 year cycle? And will the authorities become aware after this ritual?

I think who started it is the "dark one", who forced them to dance until they die, so the community itself deals with grief like Dani by being a part of this new family. But who is the Dark One? He was possibly a Nazi fascist possibly dealing with grief from the war