Does my marriage to a woman who is diabetic and barren—for the purpose of extricating her from the hell of her sister's husband's harassment—contradict the command of the Messenger of Allah, peace and blessings be upon him, to marry the affectionate and fertile?

Your marriage to this orphaned, needy girl is a good deed for which you will be rewarded. The command of the Messenger of Allah, peace and blessings be upon him, to marry affectionate and fertile women is guidance, not an obligation. Infertility cannot be definitively asserted, and if you are not blessed with children, you can marry a second wife without divorcing her, for Allah Almighty says: "Then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses."