Is a man who has committed adultery and repented forbidden from the Houris in the Afterlife? And what is the interpretation of "he is fornicated with even within the walls of his house"? And does that refer to his محارم (unmarriageable kin), and what is their sin if that is the case?

If the adulterer and others repent with sincere repentance, Allah will forgive them and expiate their evil deeds. In fact, He will, by His grace, transform their evil deeds into good deeds. The implication of this is that they will not be punished.

As for whoever persists in adultery and does not repent, they will be subject to worldly, barzakh (interim period), and otherworldly punishments. Some scholars have analogized the adulterer's deprivation of houris to the drunkard and the silk-wearer's deprivation of them in the Hereafter. Ibn Al-Qayyim supported this by stating that for everything a servant unlawfully obtains in this world, its equivalent will be withheld from him on the Day of Judgment.

As for the hadith, "Whoever commits adultery, adultery will be committed against him, even if it be with the walls of his house," it is fabricated. Even if it were authentic, its meaning would be that the adulterer's persistence in corruption might lead to the corruption of his family due to their weak faith and poor upbringing. This is among the swift punishments for those who transgress against the honor of Muslims, as Allah punishes them by violating their honor. Allah is Wise, All-Knowing, and wrongs no one.