How will God punish the aunt in the hereafter for her slander in word and deed and for swearing falsely to break up the family, sell the house, and displace her invalid brother? And will God forgive her all of that?

What the aunt did — character assassination with lies and slander, coupled with a false oath to sell the house, and inflicting harm without regard for her brother — is a grave evil. It is made worse by the fact that it occurred between relatives, severing ties and affection. Its prohibition is indicated by Allah Almighty’s saying: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin," and by the Prophet Muhammad’s (peace be upon him) saying: "Whoever says about a believer something that is not in him, Allah will make him drink from the mud of Al-Khabal until he comes out of what he said," and his saying: "The grave sins are associating partners with Allah, disobedience to parents, killing a soul, and the false oath (Al-Yamin Al-Ghamus)."

As for forgiveness, Allah is Oft-Forgiving for those who repent with a sincere repentance. Therefore, it is advisable to remind her of Allah and to warn her of His punishment, clarifying the gravity of her action. Be a support for her in her repentance and in absolving herself of injustices.