First off, send the pics of the ex to the rubbish bin ASAP, meaning from early on in the game. Keeping those pics and pinning after the cheater Gillian throws a wrench into several paths, not only Holly´s. Interestingly, for wild Holly the pics are not that important, but they fuck up a lot of paths over time. So kick them from the beginning.
Second, you need to tell Holly that she is hot/fit and have at least 5-6+ relationship points with her. The relationship points should not be a problem if you like Holly.
Additionally, if your break up with Lena was rather amiable (they stay friends), you need to make sure Emma/Perry hit it off! That is because of the room situation. There are sleeping arrangements possible depending how you played it, where you end up with someone else in the room, so Holly cannot come. The easiest way to get a guaranteed free room is to couple Emma and Perry up.