Who is more deserving of a man's kindness and good treatment, in light of the Qur’an and Sunnah: the married sister or the wife?

Both deserve kindness and good companionship. As for the sister, it is due to her kinship and because of Allah's saying: "O mankind, fear your Lord, who created you from one soul and created from it its mate and dispersed from both of them many men and women. And fear Allah, through whom you ask one another, and the wombs. Indeed, Allah is ever, over you, an Observer" (An-Nisa:1). And because of the Prophet's (peace be upon him) saying: "Whoever wishes that his provision be expanded and his life prolonged, let him maintain his ties of kinship."

As for the wife, it is due to Allah's saying: "And live with them in kindness" (An-Nisa:13). And because of the Prophet's (peace be upon him) saying: "Fear Allah concerning women, for they are your dependents." The wife has the right to maintenance and kind companionship, and the sister has the right of kinship. The wife takes precedence in obligations, while the sister takes precedence in virtue due to her kinship, as Allah says: "And those of kinship are closer to one another in the Scripture of Allah. Indeed, Allah is Knowing of all things" (Al-Anfal:75).