What is the ruling on a married man with grandchildren who secretly had intercourse with his deceased brother's wife, leading to her pregnancy, and then claimed that he had solemnized his marriage to her, knowing that she is his first wife's maternal aunt? And how should the families of the spouses deal with them if they persist in this act and do not separate?

It is Haram (definition) (forbidden) by scholarly consensus to marry a woman and her maternal aunt concurrently. If this occurs, the second marriage is invalid. If a man married the maternal aunt after her niece, with the presence of a guardian and witnesses, and he was unaware of the prohibition of concurrent marriage, then his marriage is invalid and he must cease it. In this case, sin and punishment are waived, and the child is attributed to him. However, if there is no acceptable evidence of marriage, it is considered fornication, and both individuals are subject to punishment by the legitimate authorities.