Is blood money and expiation obligatory upon the sister after her car malfunctioned suddenly, leading to an accident and the death of a person who was not wearing a seatbelt?

It is up to the investigation by the competent authorities to determine your sister's responsibility for the accident. If her fault is proven, then a blood money (Diyah) proportionate to her fault becomes obligatory upon her, to be borne by the 'Aqila (male relatives on the father's side), and the expiation for killing (Kaffarat al-Qatl) also becomes obligatory upon her. If no fault is proven against her, then there is no Diyah and no Kaffara. This life is a trial with both good and evil, as Allah Almighty says: "And We test you with evil and with good as a trial; and to Us you will be returned."