Did the mother die while angry with her daughter for having forbidden her brother from speaking to a girl, and what should the daughter do in this case?

The rights of parents are immense, and showing them piety is obligatory. It is not permissible to raise one's voice at them or to treat them poorly, even if the motive is jealousy for the sacred limits of Allah. Sincere repentance erases sins. Allah Almighty says: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the often returning [to Him]." And one can still fulfill filial piety towards parents after their death by supplicating for them, seeking forgiveness for them, giving charity on their behalf, maintaining ties of kinship through them, and honoring their friends.