2008 light novel series adapted into 2009 manga and 2011 anime.
Itsuka Tenma no Kuro Usagi (anime)
The main character's little sister (named either Yuika or Yukina) has a post-bath scene where she is drying herself with a yellow towel around while talking to her brother about the book he's reading.
The end of the episode, while it has no images, contains a dialogue between the student council president and his familiar. He suggests that she sing in the shower and then recalls that he hadn't let out the bathwater from the previous day, and suggests she uses that.