Am I obligated to feed my elderly father-in-law, for whom my husband is responsible, even if my husband believes the amount of food he provides is sufficient, and would I be sinful if my assertion is correct?

There is no harm for your husband in feeding his father twice as long as it is in the father's interest and does not harm him. However, if the father desires food at another time, and it does not harm him, then it is obligatory for the husband to feed him. As for you, there is no sin upon you if you leave off feeding him when he desires, for that is not obligatory upon you.