Did Adam (peace be upon him) exact retribution from his murderer son, or was the punishment of Qisas different at that time? And did the murderer brother marry his sister, whom the victim was supposed to marry, or did Adam prevent him?

Nothing has been mentioned in the Quran or authentic Sunnah regarding the fate of Cain after he killed his brother. All the accounts mentioned in this regard are from Israelite traditions, which are neither to be believed nor disbelieved. Among these accounts are that he was gored by a bull and died, or that the earth swallowed him up due to Adam's supplication, or that he became wild and lived in the wilderness, eating wild animals, or that he fled to the land of Eden and worshipped fire at the instigation of Satan. These accounts are not reliable, and safety lies in confining oneself to what is stated in the Quran: that the cause of the killing was envy, because Abel's offering was accepted while Cain's was not.