Must I order my family members to cover their feet during prayer and otherwise, or does the jurisprudential disagreement regarding whether a woman’s feet are ‘awrah or not leave room for no sin if I do not do so?

The summary of the fatwa is that the majority of scholars have made it obligatory for wives and female relatives to cover their feet during prayer and in front of non-mahram men. If the woman is doing this out of ignorance, then you must instruct her to do so. If she is following the opinion of scholars who permit it, then there is no sin upon her or upon you. It is preferable to instruct them to do so to avoid disagreement and as a precaution for the religion.