Double Ui

Double Ui is the Family ship between Ui Hirasawa and Yui Hirasawa from the K-On! fandom.

Canon
Yui and Ui have a close sibling bond. Yui is the older sister and Ui is the younger one, however, Yui acts like the younger sister and Ui looks like the older sister. Ui is more mature, and responsible, but Yui is lighthearted and carefree. Yui loves her younger sister very much, and Ui cooks Yui meals and cares for her when she is ill, due to their parents being busy people.

One day, Ui falls ill, and Yui is left feeling helpless. She tells her friends from the Light Music club that she can't do it, and they tell her to call them if she needs anything. Despite her lack of cooking skills, her love for Ui is enough to motivate Yui to cook something for her sister. She ends up putting an egg on top of oatmeal, but Ui appreciates the gesture.

When Yui is cooking breakfast for Ui, she makes a song dedicated to her sister, about how she feels helpless without her, and how she loves her. She performs this song at the school festival, and Ui hears it and loves the song. Yui states how she feels motivated because her sister is by her side. When Yui first joins the Light Music Club and gains independence, Ui is happy and sad at the same time.

Fanon
Since this is a family ship between two siblings, people do not ship this as much because family ships are less common than romantic ships. Most people however, think that the unbreakable bond that these two have is very sweet, and that Yui's song about her love for Ui was powerful because it accentuates the fact that Ui has always been there for Yui.

There are not that many fanfics that surround the sibling bond between the two. However Ui and Yui are present in many of them, and sometimes, their sibling bond will be a side part of the story. Fanarts of these two usually show them hugging each other, or Ui comforting Yui.

Fandom
Yui and Ui on Deviantart