Is the expiation for accidental killing obligatory upon the heirs of the deceased whose family reached a settlement for their victim's blood money, and they do not know whether their deceased fulfilled it or not, and is it permissible for them to feed the poor instead of fasting?

The grandfather was liable for the expiation of freeing a slave. If he could not find one, he had to fast for two consecutive months. Allah, the Almighty, says: "And whoever kills a believer by mistake – then the freeing of a believing slave and a compensation, to be delivered to his family, unless they [his family] forego it. But if he was from a people hostile to you and he was a believer, then the freeing of a believing slave. And if he was from a people with whom you have a treaty and he was a believer, then a compensation, to be delivered to his family, and the freeing of a believing slave. And whoever does not find [one] – then a fast for two consecutive months as repentance to Allah. And ever is Allah Knowing and Wise." {An-Nisa: 92}. It is not obligatory to perform the expiation on his behalf after his death, but whoever voluntarily fasts on his behalf will be rewarded, and the deceased's obligation will be discharged, Allah willing, according to the Prophet's saying (peace be upon him): "Whoever dies owing fasts, his guardian should fast on his behalf." Therefore, you are not obligated to fast or feed, and if you do, it is a voluntary act.