Child Minding

When I was growing up around 15yrs old mom worked long shifts at the hospital and stepfather worked in another state. I had a very difficult often irrate non verbal autistic brother and I got lumped with the the job of looking after him which included feeding and sometimes even showering him. I also had a stepsister 1yr older than me when she was at our house she would help out. On one of these occasions I was having a really hard time trying to get him to behave in the shower and my step sister came in to try and help because she wasn't always at our house he would sometimes do as she asked but not me. She started washing his back with a wash cloth while I proceed to clean up the water he'd covered the floor with when she started laughing and said omg I look across to notice his penis he had a huge erection happened before but never like this. To try annoy my sister I said I don't think he knows your his stepsister. With that she says "dare you to put it in your mouth " i freaked looked back as she's got his penis in her hand slowly stroking it while he just stands there with a empty but intreaged look on his face. Just as I said stop you can't do that he come all over her hand and wrist. We agreed that this would never happen again finished up and went to watch the movie on tv. Didn't take us long to figure out from that very point for the rest of the night he was the best behaved I'd ever seen him. Well from that day onwards the next time he wouldn't do what was expected of him. Omg I'm so ashamed of it but the next time I just pulled his towel off and I took his penis as far in mouth as I could I got used to it after about 15 seconds and it was probably only 15 seconds after that he shot the biggest load of cum in my mouth and clearly enjoyed it because he sat there and just cuddled into me on the couch and fell asleep. For about the next 15 months either my sister or myself would take turns in turning him from an irrate horror show into a quiet sleepy all be it very happy boy happy to do as his told with no issues. I know it was not the right thing to do and my sister and I never speak of it these days and I don't think (hope) there was no real harm done.