Are my brother and I considered to have caused our father's death, and is expiation for accidental killing or something similar incumbent upon us, and what is our duty towards him, and what is the greatest thing we can offer him?

We hope that your father is among those for whom Allah has decreed a good end, and we ask Allah that he be included in what the Prophet, peace and blessings be upon him, promised: "Whoever's last words are 'There is no god but Allah' will enter Paradise." We advise you to be patient and seek reward from Allah, and to strive in supplicating for him and seeking forgiveness for him, and to give charity on his behalf or dedicate the reward of righteous deeds to him. No expiation is required of you because your father made a mistake by consuming a harmful substance. Rather, you should strive in supplicating for him and seeking forgiveness for him, as this is among the greatest things that will benefit him in his grave.