Will the father and children be held accountable for not financially assisting the wife, or for the lack thereof, despite their comfortable circumstances?

The wife's maintenance is obligatory upon her husband, not upon her father. Your wife's father is not obligated to spend on her. Rather, he is obligated to equalize between his children in gifts, male and female, married or not, unless there is a valid reason for favoritism.

As for holding the father or children accountable for not spending on the wife, this stems from the obligation of their spending on her when the husband is unable. Scholars have differed on the obligation of the father's maintenance for his daughter when her husband is in financial hardship. Maintenance is not obligatory upon siblings when a capable father is present.

The husband should exert effort in earning a livelihood and seek help from Allah.