Threesome Gone Wrong

My boyfriend has been pressuring me too have a threesome for some time but I wasn’t comfortable with it.
The middle of March he, my younger sister and I went to the beach for a weekend. I was a little upset when he asked me to see if she would be interested in doing it but finally agreed to talk to her. She was a virgin so I wasn’t sure what her reaction would be.
Of course she was reluctant at first but after some discussion assuring her with me being involved I would make sure she was ok.
I’m on the pill so we bought condoms for him to use with her.
Things went well even though she was very nervous at first but we had a threesome both nights while at the beach.
Earlier this week she told me she was late. Last night I gave her a pregnancy test and sure enough somehow she got pregnant even though we took every precaution to avoid that happening.
I feel awful now that I gave into my boyfriend especially since it is my sister he knocked Up.
We haven’t told him yet so I’m not sure how he’s going to feel.
Our parents would be devastated if they found out so we are going to see what we need to do to get her an abortion.