The obligation of the deceased's guardians if he dies owing a fast

Our sister says: I had a brother who worked as a driver on a farm. One time, he collided with an elderly man, and the man died. My brother turned himself in to the police, admitting that he had hit the man, but not intentionally. My brother was released on bail, and before a year had passed, my brother was killed by a ten-year-old child. My father forgave the child, and my mother forgave him, and we all forgave the child and his family. My mother fasted for two consecutive months to expiate some of my brother's sins. What is the ruling on her fasting, and is there anything required of us regarding my brother? I ask you to please inform us?
Answer: You have done well in being lenient and forgiving, and the mother has done well in fasting on behalf of her son; because he owes an expiation, which is: freeing a believing slave. If he cannot do so, he fasts for two consecutive months. The mother has done well in fasting for two months; because obtaining a slave may be difficult at times, and the killer may be unable to do so. Slaves are rarely found, existing only in small numbers in some African countries, but they are few. If he is able to purchase a male or female slave, freeing them takes precedence over fasting. If he is not able, he fasts for two consecutive months. Since your mother fasted on his behalf, she has done well in that, based on the saying of the Prophet (peace and blessings be upon him): , meaning: his relative, and she is the closest person to him, so she has done well in that. Yes.
The presenter: It seems to me that the mother did not fast on account of the accidental killing, O Shaykh, but rather she fasted to expiate other sins? Could you guide her to something, if you please?
The Shaykh: If she fasted on behalf of her son for the fast he owed due to killing the elderly man, then this is correct. If she fasted for anything else, then this does not suffice, based on the saying of the Prophet (peace and blessings be upon him): . It is recommended for her... it is prescribed for her to fast, or her father if he is alive, or one of her brothers or one of her sisters. This is all good, based on his saying (peace and blessings be upon him): . Yes.
The presenter: May Allah reward you. If the family shares the fasting, is this permissible?
The Shaykh: This requires consecutiveness; one person must fast it. Yes.