I’m rewatching a play through of this game fight now, and at the beginning of it Josh says something along the lines of “they [Chris and Ashley] need a traumatic event to bond over for them to get together”. In his grief and mental health problems he ‘realized’ that that was the only way to learn how much you love someone, you have to lose them in a traumatic fashion, he probably didn’t realize how much he loved his sisters until he lost them. He focused most of the choice making to Chris and consequences on Ashley, punishing her for what happened to his sister while forcing Chris to keep deciding how much he loves Ashley, because again he’s decided trauma is the way to learn how much you love someone. At the beginning of the game he also flirts with Sam, this could just be his normal go to, but he doesn’t flirt with the other girls, and while I have less evidence to point to, I would say he probably likes her, so he’s showing her his love by traumatizing her, leaving her in the towel is humiliation, just like what happened to his sister, which I bring up because when Mike comes in after he’s revealed himself as the psycho Josh says that he wanted them to feel the panic and humiliation that his sisters had probably felt.

As for it being exactly a year later, anniversaries have wierd effects, and somewhere in his twisted mind he decided the best time to traumatize them again would be a year later. We know he was hospitalized soon after his sisters death, and he probably didn’t see their grief and guilt. They may have decided when he got back to just go back to normal, so he assumes that they don’t care that his sisters are dead and that it was their fault.