Am I sinful if I approve my daughter’s marriage to a young man infected with the Hepatitis C virus, which might be transmitted to her at a rate of 4%, despite my advice to her and her insistence on marrying him?

You bear no sin in marrying your daughter to a sick young man if he is religiously and morally pleasing, and the girl accepts him despite knowing of his illness. You can continuously advise her to perform Istikhara (prayer for guidance) and to avoid haste, and remind her that the truthfulness of Istikhara lies in abandoning personal choice and entrusting the matter to Allah. Al-Qurtubi said: "He should empty his heart of all thoughts so that he is not inclined towards any particular matter."