Also märkäkorva lappalainen (wet-behind-the-ears Lapp) who has humped, not only his mother and sister, also all the mares in his household. Lemminkäinen finds him so disgusting he doesn't bother to kill him.
In many versions, Morgause is Arthur's half-sister. Their son and nephew, Mordred, eventually destroys Arthur's kingdom.
The Questing Beast was said to be born from a princess who lusted after her brother, and slept with a demon who promised to make him love her back. Instead the demon made her hate her brother, and when she became pregnant she accused him of raping her, prompting her father to execute him. Before he died the prince said that if he was innocent, it would be proved by his sister giving birth to a monster.
In Norse Mythology (at least according to Heimskringla), marriage and breeding between brother and sister were common amongst the Vanir before their alliance with the Aesir. The twins Freyja and Freyr were children of Njord with his unnamed sister. Loki in the Poetic Edda also once accused Freyja and Freyr of sleeping with each other, though it's not clear whether this is more than slander. Through in the following stanza Njord defends Freyja, but it's for the infidelity-claim. The incest part is not mentioned. It does make you wonder through...
Aztec Mythology: In the myth of creation, Quetzalcóatl was a celibate god. Then he got drunk after a fight with his brother/alter ego Tezcatlipoca and had sex with his sister.
Classical Mythology: The whole pantheon is filled with incest of just about every conceivable combination. Brother-Sister Incest is probably the least strange of it.
Siblings Cronus and Rhea got together had 6 kids: Hestia, Hades, Hera, Posiden, Demeter, and Zeus. Those kids proceed to get together with each other:
Zeus also had a daughter (Persephone) with his sister Demeter.
There was also a myth about Poseidon's short affair with Demeter which may or may not have been consensual. (Whether it was seduction or rape was unclear, something that is often the case in these myths.)
Byblis falls in love with her brother Caunus, and defends their relationship by pointing out how many immortals have had incestuous relationships. Their story was later retold by Ovid.
According to the Odyssey, Aeolus, Keeper of the Winds, married his six sons to his six daughters.
In another story, incompatible with the above, Aeolus' children Macareus and Canace had a secret affair. When Canace gave birth to a son, their relationship was revealed. Aeolus exposed the child, sentenced Canace to death and Macareus killed himself.
Hindu Mythology: Averted in the Rigveda. Twins Yami and Yama are the first created mortals, and Yami attempts to seduce Yama so they may continue the human race. Yama refuses on the grounds that she's his sister so that's just wrong.
Polynesian Mythology: In ancient Hawaiian myth, the divine couple who gave birth to the Hawaiian islands were either siblings or half-siblings. They also had a daughter who grew up to be so beautiful that her father begun a relationship with her and fathered two more kids. This became the basis for a practice known as pi'o, intentional incestuous mating amongst the ruling class. Extensive genealogies were kept in order to produce the most inbred (and thus, godly) chiefs possible. The commoners were forbidden to do this out of fears that they would start producing children with chieflike levels of mana.
Alchemy has many incest symbols, especially the hierosgamos or coniunctio ("sacred marriage" or "union"), a chemical wedding of male and female, brother and sister. The rebus is often shown as an incestuous brother and sister, portrayed as a union of Sol and Luna, sun and moon.
Nut and Geb are also technically siblings and in love with each other, but are forced to remain separate to maintain the order of the cosmos. They had four kids (Nephthys, Set, Isis, Osiris) who paired up with each other:
Isis/Osiris. In some versions of the mythos, Isis gives birth to reincarnations of herself and her husband who mate even before birth—may sound icky at first, but this is in all probability a fertility myth inspired by the constant rebirth of plants and harvest.
Chinese Mythology is unclear as to whether human beings originated from divine incest: The first god Fu Xi and the first goddess Nu Wa, both of them beings with human torsos and snake tails, are at once brother and sister as well as husband and wife. It is usually stated that the first human are created as clay figurines by Nu Wa, but both deities are credited as the creators of human.
Japanese Mythology have the first deities Izanami and Izanagi. Some versions of the myth also have the sun goddess Amaterasu married to her brother Tsukuyomi.
Adam and Eve. Since humanity is still young and vital, it's a case of Incest Is Relative until more careful selection is possible or even necessary.
Eve was made from Adam's rib, so unless God worked some additional magic, they were brother/sister in a cloning sense.
Adam's and Eve's children by the more literal readings could only have had other family members to mate with but its never said explicitly. Cain's wife who is arguably the 5th human mentioned but comes from nowhere. After Cain killed Abel, he went into exile away from his family, but he went on to marry the unnamed lady. The fifth chapter states Adam and Eve had other sons and daughters, so she could've been one of them. Alternatively, some think the implication is that there were other people created other than Adam and Eve, and they were just the first.
The punitive Great Flood leaves only four married couples alive — three of the men are brothers, and the fourth couple are their parents. The following children would have no one to marry but their cousins or siblings. As Genesis also claims that people lived an average of 400 years back in those days, the much shorter lifespans following the Flood can be taken as a result of that tragically narrowed gene pool. This is all consistent with the Bible's theme of various cataclysms as a consequence of man's pride leading to repeated falls.
Not only for humans. Aside from bringing seven pairs of a few species designated acceptable for sacrifice (and any domesticated animals the family might have had), the Biblical account puts "every air-breathing animal on the planet" through the same genetic bottleneck.
In Genesis 20:12, Abraham says that his wife Sarah is his half-sister, the daughter of his father. There is some controversy about this; some Jewish sources, such as Rashi, note that since at one point Abraham calls his nephew Lot his "brother," Sarah may have been another name for Abraham's niece Icsah. Uncle/niece marriages are considered valid under Biblical law.
Other than the story of Amnon and Tamar, however, all of these incidents took place before the books of Leviticus and Deuteronomy, which specifically outlawed these practices, were written. The very word "incest" is a term that has been retro-actively applied, coming as it does from Latin, the language of ancient (yet still much younger than Israel) Rome. Though there are indications that the Law was already known to some extent before Moses wrote it down (e.g., Noah knew which animals were clean before Leviticus 11 was written).
In Zoroastrianism there is a concept known as xwēdōdah, or the holy union of father and daughter/mother and son/brother and sister. It was and still is considered by the religion as the most pious acts one can perform as a Zoroastrian for reasons that are far too complicated to explain in a simple entry. However, since nothing in the religion requires it and most people simply don't want to marry someone in their family, it very rarely happened outside of royalty.
The Pearl Of Great Price is one of the four standard works in LDS (Mormon) canon scripture. It says that the sons and daughters of Adam and Eve began to "divide two and two in the land ... and they also begat sons and daughters" meaning it's explicitly Mormon doctrine that everyone is a product of sibling incest. While this neatly explains where Cain's wife came from (she was his sister) it...brings up other issues.