Waalaykum Salam Brother.
Before I come to your query, I'd like to mention that Haram relation is not only the physical relation that a woman and man have! To have an opposite gender friend is also haram. In that case, this sister has had a few haram relations already, even if they didn't involve any physical contact. Marriage is a very important step in life. And it entirely Begins with finding a correct spouse.
According to the Islamic law, (as collected from Sayyid Sistani's office) it isn't Wajib on you to marry her if you're doing it after feeling sorry for her, and you're also not obliged to call it off if you think it's the right thing to do. The only thing however you should make sure of is that, it shouldn't effect your life after marriage. And that there should be some sort of guarantee that, such relation will not continue. I would suggest you further take some counselling from experience people, to make a perfect decision.