I'm fairly certain the event at Ivy's place where she, and possibly Lena, have sex with Holly happens after the book weekend. If Holly is with Ian at that point, Ivy gets a booty call and sends Lena and Holly home early. There is an earlier get together at Ivy's place that happens before the book weekend, but the most that can happen there is either Ivy or Lena can kiss Holly. This does not open up later sex scenes, however, just later text where Lena reminisces about the kiss.
As far as I can tell, the only way Lena can have sex with Holly is if neither of them are sleeping with Ian.