My best mate might of got my sister pregnant but she wont take the test.?

My best mate is 15 and my little sister is 13, 2 weeks ago it was 9 at night and my mum and dad had gone out and was going to be home at half 10. Me and my mate were going to meet someone so i left him downstairs watching tv and my sister was in her room. I had music on fullblast but heard someone coming upstairs i thort it might just be my sister went to get a drink or something. So i didnt check. I was about to go in my sisters room and heard moaning come in from her i ran downstairs to get my mate but he wernt there, im very protective over my sister so i walked in and the quilt was going up and down and my mate was giving hickeys on her neck and she had her eyes closed moaning. I shouted at him to get out and he didnt listen to me and my sister started crying go can you go away please. My mate got off her and i punched him in the face he fell down and got up and goes woah mate anger. i go i just saw you fuking shagging my underage sister. He jusst walked out the house. my sister was under the covers crying. normaly shes realy protective about people seeing her naked but she didnt care and hugged me saying she was sorry. I asked her what happend and she said she got out the shower and he came into her room and she was naked and he started touching her and all she was doing was stairing not noing what to do and then he put it in. He adimited to it. yesterday i got a call from my girlfriend saying that my sister asked her to buy her a pregnancy test but told me not to tell you and she brought her one but gave it to me. I went into her room and she was watching tv. i sat by her and goes so when do you want to take this. i handed it to her and she took it and thew it at the wall im not taking it she shouted at me i dont care and i dont want to find out. I realy need her to take it so i can help her but she wont how can i get her to or trick her into it im just thinking of her i need to no how it works and if i can get her to take it without relising