First we have what is probably is the most in famous pairing in Anime in recent times Kyousuke and Kirino Kosaka from OreImo who actually had a relationship with each other. Some of the pairings below may not be related by blood, but I'm not saying who to avoid spoilers. In most places incest is considered a taboo in modern society, and Yuzuki Kurebayashi from Selector Infected Wixoss just wants to break that taboo and have her wish granted — to be in a relationship with her brother Kazuki without society looking down on them. She sometimes acts like a stalker, and is jealous of any girls that show affection to Tomoya, but acts like a big sister when the situation arises. Her brother Akito on the other hand just thinks she is annoying ehmmm This is some kind of subversion since in this relationship the Sister is the older of the siblings, but man what an interesting older sister, outspoken and passionate, she immediately falls in love with Tomoya and is overly protective of him. In real life, relationships between sisters and brothers are a big taboo.
Julie. Age: 20.
Also in the words of Koda89 "That triangle mouth!
Millie. Age: 22.
Their relationship got so close that they eventually fell in love with one another and then made love, breaking one of society's biggest taboos. They may not be as legendary as some others but they also did " that ". Let me know in the comments below.