What an awful situation! And may I also say what an a-hole!
I fell pg at 18, when I discovered I was pg we had only been together 6 months. His family were awful, but I wouldn't be swayed.
My DF dropped his burger all over himself when I blurted it out. It was tough, we had no idea if we even wanted to be together long term. But we decided after a month of soul searching that we'd give it a good go. Termination was never an option for me so it was more would we do this together or seperately.
We moved in together and are still together 5 years later- we are actually getting married next month We have also had another baby we have 3 angels together too.
Good luck to your sister- I will pray for her and you also. Lots of love xxxx