Is it considered a sin for a mother to refuse her niece's son entry into her house, after he attempted to assault her daughter's son?

It is permissible to shun a young man and prevent him from entering the house to avoid his evil, if he persists in his disgraceful act. However, if he repents and reforms himself, then it is not permissible to shun him; because he has a right to kinship, being one of the relatives whose ties must be maintained, and because one who repents from a sin is like one who has no sin, for Allah Almighty says: "And as for those who commit indecency among you, punish them both. But if they repent and amend, then leave them alone. Indeed, Allah is ever Accepting of repentance, Most Merciful."