Is the wife obligated to serve her husband's parents, especially if their daughter and a maid are present in the house, and would she be sinful if she does not do so?

You are not obligated to serve your husband's parents; rather, they are obligated to serve themselves. There is no blame upon you for refraining from doing so, unless your volunteering to serve them is considered good companionship and noble character that will weigh heavily in the scales [of good deeds].