I Lost My Boyfriend To My Sister

My little sister lives with me. She's 17 and will be a senior in high school. Our mother is an alcoholic, unemployed, in and out of women's shelters, living off of friends, no longer accepted by relatives, including me. I have a job, work to have a life, my sister was awarded to me as her guardian.

Two weekends ago my boyfriend was over and he kissed her. He told her she was beautiful and 'one day you're going to have my babies'. I woke up and found her in bed with us, cuddled up under him. He swore he didn't 'fuck' her but he did. She had his sticky cum inside her legs, I checked her and she admitted to it. He then admitted to a little bit of hiding the weenie in her.

I kicked him out. She won't talk to me, she used her allowance to Uber to his apartment, he showed her she was welcome and treated her to a full on lovemaking session, a full no holes bared love making.

I am 8 weeks pregnant, I haven't told him or her. My happiest moment, now I have terminate my pregnancy.