How can one rid oneself of the sin of taking money from one's mother without her knowledge after her death, knowing that the questioner is the sole heir? And is the mother's death a punishment from God?

It is permissible for a child to take from the wealth of their father or mother, upon whom their maintenance is obligatory, to the extent of their essential needs, even without their knowledge, if they are unable to obtain maintenance through other means. This is analogous to the story of Hind bint Utbah. However, if the taking is for expansion or without need, it is not permissible, and repentance is required, along with returning the money to its owner or their heirs.