Praise be to Allaah.
Your sister, you and your family should have paid attention to what your sister committed of sin, and should have instructed her to repent sincerely to Allah, may He be glorified and exalted, so as to cleanse herself of the taint of disobedience and zina (fornication) into which she has fallen with this evil young man or anyone else. And you should have cut off all the means that lead to this sin, and made her wear hijab, and prevented her from being alone with men or mixing with them, or doing anything else that leads to this evil action. So strive to keep him away from her and keep her away from him by all possible means.
Then after that, you have to think about this child and the future of your sister. All of this is the consequences of disobedience and sin; the sinner has to bear this burden and shame in this world, so how about the punishment that is with Allah?
One of the wise men said: If you get tired of doing righteous deeds, the tiredness will disappear but the righteous deeds will remain. But if you enjoy committing sin, the pleasure will disappear but the sin will remain!
What your sister is trying to do of protecting the foetus is what she has to do and what you all have to do, not only in the hope that this evildoer will come back to her, because it does not seem that he wants to repent or that he wants to marry her in a legitimate, shar‘i (legal) way after fulfilling his desires with her in a haraam (unlawful) way. Rather you have to protect the foetus lest you try to deal with one crime by means of another and you harm a soul that has not done anything wrong, because the sin is on those who committed zina in the first place, then wanted to harm this soul, either by killing it or by neglecting it and leaving it in the street or putting it in an orphanage, as some people do. Allah knows best about what would happen to it, but it is most likely that it would be brought up in a kaafir (non-Muslim) orphanage or it would be raised by a kaafir family, whether Jewish or Christian or of some other religion, and it would follow them in their Judaism or Christianity or whatever religion they follow. This is the worst and most abhorrent crime against this soul, and it is worse than killing it, Allah forbid.
See also the answers to questions no. 13331 and 117.
With regard to this person saying that he is prepared to go to Hell and he does not care about that, these are words that cannot be spoken by someone who believes in Allah and the Last Day, and who fears the meeting with his Lord. Allah will take care of him. Let the wise man look and learn a lesson, how people are deceived by this world and its people and how they dare to throw themselves into the Fire of Hell, and they do not care. May Allah the Most Generous keep us safe by His grace and bounty.
In such circumstances the parents should continue showing kindness and taking care of your sister, whilst calling her to repent and mend her ways, and they should try to prevent her leaving the house or running away from home, lest that make matters worse.
We ask Allah to bestow His kindness upon you and set your affairs straight.
And Allah knows best.