What is the ruling on one who intentionally breaks a votive fast? And what is incumbent upon one who caused the death of his two sisters in a car accident, in terms of fasting or charity?

Undertaking a vow is disliked, but it is obligatory to fulfill it if it is a vow of obedience. So, whoever vows to fast for a day and then breaks their fast must fast another day in its place, whether the day was unspecified or specified, though there is a difference of opinion regarding making up a specified day.

It is not permissible to invalidate a vow-fast, due to the consequence of nullifying an act of worship.

If traffic accidents result in fatalities and the cause of the accident is attributable to the driver due to speeding or negligence in vehicle maintenance, then the killing is considered an error. In this case, the blood money (diyya) is obligatory upon the driver's 'aqila (male agnatic relatives), and expiation (kaffara) is obligatory upon the driver, which is the freeing of a believing slave. If one cannot find a slave, then fasting for two consecutive months for each person killed is required.

However, if the driver was adhering to traffic rules and not negligent in safety procedures, then neither blood money is due from his 'aqila nor is expiation obligatory upon him.