I have not watched this movie, but only the trailer, and I remember I found it fascinating and wanted to watch it later. Sadly, I have no clue anymore what the name was or the proper plot really.
It seemed like a young adult fiction movie and was probably from the POV of the guy that was befriended by the siblings. From what I remember, it seemed like the guy was pulled into the more exciting life of the siblings or so (so like the manic pixie dream girl trope but with siblings). I think there was a scene where they were running through something like a museum or hallway with statues or something, laughing. And I think there also was a scene in it that indicated that it turned sexual between all of them or that there was more going on between the siblings.
This is exactly what I was looking for! Thank you so much:)