All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is obligatory for that young man and that girl to sincerely repent to Allah, and avoid all that can lead them to commit Zina (fornication) again, such as sinful relationships, being in forbidden seclusion (together), and so on. For more benefit, please refer to Fatwa 86527.
The jurists agreed that abortion is forbidden after the soul is breathed into the fetus except for necessity, such as fearing for the life of his mother.
However, they differed in opinion about abortion before the soul is breathed into the fetus; some permitted it before forty days, and some permitted it before the soul is breathed into the fetus.
The view we adopt here in Islamweb is that it is not permissible at all, because doing so is a destruction of the progeny and corruption in the land, and Allah does not love corruption. Additionally, it is an unjust aggression and transgression on a created soul.
Pregnancy which results from Zina (fornication or adultery) is absolutely not a justification to undergo abortion, regardless of the psychological and social obstacles that may result of it. This conduct (abortion) does not remove the crime of Zina, but it rather adds another crime.
The child (born out of illicit relationship) is attributed to his mother and not to the fornicator according to the view of the majority of the scholars. However, some scholars are of the view that the child may be traced back to the adulterer/fornicator is he accepts tracing the child back to him.
If they both repent to Allah, then that young man may marry that girl, and her guardian is not permitted to prevent her from marrying him without a legitimate reason. If he does so [i.e. if he prevents her], then she may take her case to the competent authorities specialized dealing with the matters of Muslims in non-Muslim countries, so that they would remove harm off her and marry her off to a suitable man, in case her guardian insists on preventing her without a legitimate justification.
Finally, we draw your attention to the seriousness of the stubbornness of the guardian and his insistence not to marry off the girl under his guardianship for baseless reasons, such as what is mentioned in the question that that girl has an older sister who is not yet married. The consequences of such stubbornness are evil, and what happened to that girl is a proof of that.
For more benefit, please refer to Fataawa 95615, 314959, 131279, 86903, and 81160.
Allah knows best.