Praise be to Allah.
This relationship that developed between the young man and the young woman, which usually starts with a look and infatuation, and often ends with immoral actions, is a haraam and sinful relationship which can only lead to evil, corruption and temptation. Allah, may He be glorified and exalted, says (interpretation of the meaning):
“And come not near to the unlawful sexual intercourse. Verily, it is a Fahishah (i.e. anything that transgresses its limits (a great sin)), and an evil way (that leads one to Hell unless Allah forgives him)”
Shaykh as-Sa‘di (may Allah have mercy on him) said:
The prohibition on coming near it is more eloquent than simply prohibiting this action, because that includes the prohibition on all the things that lead to it and promote it. The one who plays with fire will soon get burnt, especially with regard to this matter which many people have a strong desire for.
End quote from Tafseer as-Sa‘di, p. 457
For a young man to get to know a girl who is not his mahram, and for them to fall in love with one another, and what that leads to of meetings and conversations and strong emotions and so on – all of these are among the most easily accessible doorways to mischief and fitnah. Anyone who has ears to hear or eyes to see will know that for certain.
The issue here is not a case of magic that was done by his sister, or anything else; rather the issue is that this relationship was built on a false foundation from the outset. The first step to putting things right, and the most essential obligatory duty for both of them in this case is to repent to Allah; both the young man and the young woman should repent from that illicit relationship, then they should turn over a new page with Allah. One of the conditions of repentance for the sinner is that he should regret the sin that has passed, give it up, and resolve not to go back to it again. All of these steps mean that each of them is required to cut off ties with the other, because this is an invalid relationship that was based on haraam.
There is no room here for praying istikhaarah, because salaat al-istikhaarah is only prescribed concerning permissible matters that a person may be confused about and not know which is better for him. With regard to obligatory or mustahabb (encouraged) matters, there is no istikhaarah for them at all, because it is enjoined to do them according to Islam. By the same token, there is no istikhaarah for haraam and makrooh (disliked) matters either, because it is forbidden to do them according to Islam.
See the answer to question no. 11981
Once it is understood that there is no room for istikhaarah in this case, according to Islam, and that what is required from both parties is to repent sincerely to Allah and put an end to the bad relationship between them, any wise person will realise that holding onto far-fetched notions and relying on a human being who has gone away is foolish and is contrary to what is in one’s best interests in both religious and worldly terms. Hence it is said: Whoever referred you to someone who is absent has not been fair to you. This young man turned away from marrying that girl, even though he lived close to her in the same city, so how can she wait for him after he has left her and gone far away?
If both of them repent, and cut off ties between them, then after that he wants to marry her, there is nothing wrong with that, but that is on condition that you do not wait for him; rather you should erase that dark page from your life, and start a new page, in the hopes that Allah will accept your repentance, help you to carry on with your life, conceal your past mistake, and give you some one better than him.
And Allah knows best.