All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, you are both obliged to immediately repent from this illegal relationship; for more benefit, please refer to Fataawa 81356 and 86527.
Secondly, if this girl repents, then it is permissible for you to marry her, in which case, you should forget what she did in the past. What is important now is that she has become religious; for more benefit, please refer to Fatwa 86667.
However, if you know that you cannot forget her past, then you should forget about marrying her in order to avoid bad consequences, like disputes, distress and perhaps even divorce.
Thirdly, it is not permissible for you to inform your family about what this girl did in the past. She should have concealed herself about what she did and not inform you about it, because this is exposing one’s sin and this is forbidden. For more benefit, please refer to Fataawa 84052 and 83573.
Fourthly, if marriage takes place, both of you should continue your life as if nothing had happened, and each one of you should fulfill as best as he/she can the right of the other. For more benefit on the rights of the spouses on each other, please refer to Fataawa 85308 and 83157.
Fifthly, if your family members, especially your parents, do not agree to your marriage to this girl, then you should try to convince them about it. If your parents insist on refusing, you are obliged to obey them unless you fear to be tempted by her and commit with her what Allaah has forbidden, in which case, you are permitted to marry her; for more benefit, please refer to Fatwa 106420. After that, you should endeavor to please your parents.
Allaah Knows best.