The beauty of a comics multiverse is that it opens the door for stories which simply couldn't be told within a superhero universe's main continuity. However, these alternate universe stories can include some questionable plotlines, such as the incestuous relationship between Wanda and Pietro Lensherr in the Ultimate Universe. In the early 2000s, Marvel boldly reimagined Scarlet Witch and Quicksilver by giving them a sibling romance that was strikingly similar to the one shared by Cersei and Jaime Lannister in HBO's Game of Thrones.
The family dynamic of the Lensherrs is the key to understanding what caused the twins' incest. Their childhood played an important role in shaping their relationship, although it still does not justify what happened between them any more than it does for the Lannisters.
Scarlet Witch and Quicksilver's Ultimate Marvel Relationship
One of the catalysts for what ultimately transpired between Wanda and Pietro can be ascribed to their father, Magneto. The Master of Magnetism was extremely abusive towards Pietro, to the point where he cried himself to sleep night after night. With his mother out of the picture and his father the source of his pain, the only person Pietro could turn to for any sort of comfort was Wanda.
Wanda soon went from being Pietro's mother figure to his love interest. Quicksilver's level of obsession with the Scarlet Witch therefore became far greater than anything experienced by the Lannisters.
The Evolution of Scarlet Witch and Quicksilver's Inappropriate Relationship
The mutant siblings become inseparable, acting fiercely to protect one another from any perceived threat. There were subtle hints about the true nature of their relationship at first, but when Captain America suggested that Wanda should change her costume to something less revealing, Pietro's violent reaction hinted that there was more to their relationship than met the eye. This suspicion was then confirmed by Wasp, who made the details of their relationship more than speculation.
Shortly after being confronted by Captain America, Wanda was assassinated by an Ultron unit that she had inadvertently altered to become obsessed with her. The robot regarded Quicksilver as a rival and elected to terminate Scarlet Witch rather than seeing her in love with someone else, thus ending one of the most controversial relationships in comics.