Am I obligated to visit my father at the home of his second wife, whom I detest, given that I am prepared to visit him anywhere else, and would I be sinful if I refuse to visit her?

You must obey your father by visiting him at his home. Your dislike for his wife is not a valid reason to refrain from visiting, as your father is the intended recipient of the visit. Enduring slight hardship for the sake of honoring your father is an obligation, and it is among the most important causes for Allah's pleasure and entering Paradise, as the Prophet, peace and blessings be upon him, said: "The father is the middle gate of Paradise." Obedience to parents is obligatory in matters that do not involve disobedience [to Allah], even if they are disobedient [to Allah], as long as no harm results from it. Visiting the stepmother is commendable due to its aspect of honoring the father, and obedience to him is obligatory if he commands you to visit her, as long as no harm is involved.