I strongly believe that it is not good to fornicate, but I did this some days ago. I've regretted what I've done. I want to start dating a woman whom I would like to marry, but the fact is that she is the sister to the girl with whom I had sex. The woman I want to marry knows nothing about what has gone on between me and her sister. Meanwhile the girl I had sex with knows that her sister is in love with me and has given me the go-ahead to be with her, if I want this. She had warned me not to tell her sister about anything. I sincerely don't want to fornicate anymore with this girl. However, if I must marry the girl I had sex with, I can abstain until I marry her.
My question is: Am I allowed to marry the woman since I had already committed fornication with her own sister? If I should, do I have to keep the secret? If I shouldn't, how do I explain this to her so we can separate?
You made a mess of this situation with your sin. Regardless of what the girl you had sex with says, this sin of yours will very likely come to light at the worse possible time, such as when the two sisters are having an argument. I'm trying to figure out why the girl desired to have sex with the boy she knew her sister loved. Any motive I can think of would indicate the two sisters don't get along with each other. Therefore, the only solution is to tell the girl you would like to marry what you've done. It will likely cause her to reject you. She may not speak to you again for months, if ever. However, if she does eventually decide that you are worth having as a husband, then she will do so knowing your flaws. In other words, this will have to be her decision.
Once you mention what your sin is, she will stop listening, so you are best off starting out explaining that you've done a horrible thing that you completely regret and wished never happened. That way she might remember that you have repented of your action.
You also have the choice of marrying the girl you've already had sex with. Her sister will hate her for stealing her boyfriend away from her, but that wound will more likely heal in time.
The real problem, however, is that you have shown yourself to be a poor choice for a husband. After all, if a guy was willing to have sex with his girlfriend's sister, it leaves question of what else you might do, seeing that you show little restraint on your behavior. It seems to me that your best choice is break up with both sisters, get yourself right with God, and then find someone else to marry.
Thank you very much for the explanation. I have perfectly understood everything, except that I don't know if there is any biblical quotations that will support my act of getting married to the lady I've not have sex with. Frankly, I want to know if it is not a sin against God to get married to the lady I've not have sex with.
In the Old Testament, there was a law that forbade a man from having sex with his wife's sister. "You shall not marry a woman in addition to her sister as a rival while she is alive, to uncover her nakedness" (Leviticus 18:18). The Old Testament law is no longer in effect, but the New Testament would forbid this because it only allows one wife (Matthew 19:4-6). Besides, it isn't quite your situation since you are looking to marry the sister of the girl you had sex with.
However, some things can be technically legal but unwise. That is what I tried to point out to you. It isn't a sin to marry the sister, but it is very likely unwise.