Very nice- but I think if Koishi's hands made indentations or her dick made a bulge in the stomach rather than a transparent ghost-like penis.. it would be better for teasing.
Something made with the full Transparent 0 in mind- perhaps the skirt of the dress lifts up as Koishi pushes against Rin or one of her breasts(the one being kneaded) shakes more vigorously than the other. But it's already a very complex animation. Great work