In the film, remember how Rebecca threw a feral cat outdoors when Eileen arrived for the Christmas dinner? We later learn why the cat was upset about Rebecca. Because she was an intruder.
In the film, Eileen's father says that Eileen's sister Joanie smells like a woman and, while drunk, he caresses Eileen's thigh while mumbling Joanie's name and shushing her objections. With these clues that he probably abused Joanie, it is no wonder that Joanie won't talk to him and that Eileen phantasizes about killing him, is drawn to Lee, who killed his father, and wants to blame Lee's mother's death on her own father.
The ironic twist is that Rebecca tries to uncover and heal people's trauma that nobody else could, and repeatedly attempts to "read" Eileen, but totally underestimates just how trigger-happy (and traumatized) Eileen really is.
Rebecca's party escapades seem reckless, but we later learn that that was the night after she found out about Lee's true story. She was just trying to cope and to connect with someone who would make her feel understood in a small town where she just arrived alone.
