Is the wife considered sinful for her silence regarding the murder of her mother-in-law, who died from a brain hemorrhage after her daughter struck her, given that the wife was aware of the incident, heard the victim's screams, and heard the perpetrator's confession?

The questioner heard a scream but saw nothing, so she cannot testify. Such a killing is usually accidental, not intentional, and thus requires blood money (diyah) rather than retribution (qisas). Some jurists have stated that concealing the killer—even with a lie—is a form of benevolence, as Al-Babarti said. Most important is to advise the girl to offer sincere repentance and to increase her good deeds, for disobeying mothers is among the gravest sins.