How authentic is the following Hadith: "A group of people agreed to commit incest with their sisters and mothers, and they did so. So Allah sent upon them an insect which killed them. One of them escaped, and a message came to him. He opened it, and the insect came out of it and killed him"?

No evidence has been found to suggest that this report is a hadith; rather, it is a story mentioned by some preachers. Incest is a greater sin than fornication with non-mahrams, due to the breaking of kinship ties and opposition to natural human disposition (fitra) involved. Some scholars have gone so far as to say that one who commits incest should be executed without exception, whether married or unmarried. This is a narration from Imam Ahmad, and it was chosen by Ibn al-Qayyim and Sheikh Ibn Uthaymeen, who cited the hadith of Al-Bara' bin Azib concerning the killing of one who marries his father's wife, and the hadith of Ibn Abbas: "Whoever has sexual intercourse with a mahram woman, kill him." The majority, however, hold that he should be given the prescribed punishment for fornication (stoning for the married person and lashing for the unmarried person).