Wait...so Why? [Contains Spoilers]

Did Kaori sleep with her sister's boyfriend? The scene before that we see Kaori on the balcony, and Miyuki's boyfriend standing by the doorway of the apartment. We see Kaori quickly walk to him and for an instant, she turns into Miyuki. But she changes back to Kaori as she jumps into the guy's arms...then suddenly they're in bed together? And they just met...and afterwords they continue looking for Miyuki...that was really odd to me. Anyone care to enlighten me? =)
I was thinking it was either Miyuki in Kaori's body, but it looks to me that Kaori is in control of herself...