What is the ruling on my refusal of my sister's marriage to a young man who is not religiously committed and works in a place where there are abominations, and am I sinful for allowing my sister to study at a mixed-gender university, which led to her attachment to this young man, and what is the way to convince her that this marriage is not suitable?

Your sister's enrollment in a mixed-gender college is a means to fall into forbidden acts, and what happened was the least of the harms. This young man who engages in unlawful work and earns from it is not suitable for your committed sister. The Messenger of Allah, peace and blessings be upon him, said: "If there comes to you one whose religion and character you approve, then marry him; otherwise, there will be tribulation on earth and widespread corruption." We advise turning away from this young man and waiting for one who possesses [good] religion and character.