SOD has been granting the wishes of older sisters who want to get closer to their beloved younger brothers, but this time, they are going to help out an older sister who wants to get closer to her younger brother on a family hot spring trip during summer vacation! It's been years since they've had a mixed bath with their family... Even though their parents are nearby, will the older sister end up committing "incest" with her younger brother, even though she knows it's wrong?