Currently you can have sex with both Yuna and her friend that stays over for sleepover Kaede, unlock her first at the end of the party, where you have a chance to convince her to stay, then she will be in Yunas room that same night just after the party, on the right side of the room and will be there everytime it rains during the night (you can use rain stick in the morning to make it rain that night and spawning Kaede)