Fucking mates sister

I have become good mates with a lad in school who was being bullied. I've seen him many times getting picked on but that day i stopped it and since then we been hanging out. as I'm popular in school he's been left alone.it felt like to keep the friendship he was trying to give me stuff and i refused of course. as i live far from our school he comes to my neighbourhood which i find out he has a aunt that lives up the road from me and in those first two weeks i basically knew everything about him.with the teachers going on strike a few months ago we arranged to meet up near my nans house as it's halfway from our homes,he turned up with a girl that is fit.i asked who is she and it turns out it's his 13 yo sister.we was hanging out and he said his sister likes me and that's the reason she came along,I pulled him to the side and he's like it's cool you can fuck her she wants it.i looked over at her laying on the grass and asked him if he was serious when he said that to her she got up put her hand up her skirt and took off her knickers.i had keys to my nans house for emergencys, i took her there without my mate of course straight to her spare room and fucked this 13yo.my mate i find out went off to his aunt i messaged to say i will let know when we was done.i fucked her but she was different i wanted more than just a one off fuck ,I'm 16 and can't stop fucking her every chance i get it's like a drug I'm hooked and can't give it up.