What should I do as expiation for my sin toward my deceased mother, who died due to blood poisoning? Will I be held accountable for her prayers and fasting? And was my negligence the cause of her death?

We ask God to forgive your mother and have mercy on her. You should not burden yourself with the responsibility of having caused her death, for one's appointed time of death neither advances nor recedes. If there was any shortcoming on your part, seek God's forgiveness. The obligation of fasting and prayer upon her is based on her being of sound mind; if she was senile or insane, she is not held accountable. Otherwise, prayers are not made up for her, but rather, forgiveness and mercy are sought for her through supplication. As for obligatory fasting, a poor person should be fed for each day she missed. If you fast on her behalf, that will suffice.