Pelle killed Dani's family?

It's certainly possible. I thought it was weird how she had duck tapped the hose to her face which could have been done by force. However, he still needed Chris to invite her on the trip. There was always the possibility that he doesn't invite her or more likely that he breaks up with her prior to it since they were clearing having problems. I think that he just had a crush on her and the theory is a bit of a stretch.

I agree it's possible. On a related note I'm not sure Pelle loved her in the way one normally views love. I think instead that she may have been his astrological match similar to how the red haired woman was seen as a fit pairing with Christian. Almost an arranged couple of sorts but based not on wealth or circumstance yet some sort of celestial or spiritual basis. Like, they were the right energies to be joined together.

Agree it is possible. There was a photo of Dani on a nightstand in the parents' bedroom which was "crowned" with flowers very similar to the ones the ladies were wearing during the maypole dance. Odd.

Oh! Bad-ass theory!

Yes I agree. I thought he was flirting with her very early into the movie. I expected the ending to some how weave in the death of her family and the connection.

No.

The director actually confirmed it was the sister but that he doesn't like killing peoples' imagination.

No, not all lol. He starts to groom her as soon as the group finds out she's being invited to Sweden with them. He definitely likes her, but also knows she would be a great choice for a new may queen. The guy literally thinks by giving them a new outsider, he'll be sacrificed and go to the next level of that cult.