Unexpected Threesome Gone Bad

A few weeks ago me, my boyfriend and my sister were in my room horsing around and we started having sex in front of my sister. She got pissed saying it wasn’t fair us doing it in front of her so I asked her if she wanted to join. I thought she would say no because she had never done anything but she jumped at the chance. It was actually very exciting watching him do my sister for her first time and she definitely enjoyed it . Problem is she enjoyed it a little too much and she got pregnant. Now she doesn’t want to get a abortion which is causing problems for me and my boyfriend.