I posted this in answer to girl and her brother. I knew one other boy who had sex with his sister. She had sex with us both few times. She was older by 3 or 4 years. I think she used us to practice for her BF. We were not get lovers, we provided her entertainment. I did this in my early and mid teens and like most boys, it was real pussy and the sister part was easy to get by. I did not do my own sister, and doubt I would if she ask. I did feel guilty but like most others I got over it.

This was my response.
friend of mine had a sister year younger. He showed me hole in her closet. door not closed we watched her dress. We watched her finger and using her hair brush handle. We jerked off and watched in amazement.
he told me he had sex with her few times. I ask her, she admitted it. I did not believe this. Next day just us 3 and he pulled her clothes off and she sucked his cock. She waved me over and sucked mine. She laid back and guided my cock in her and with in a few strokes I came in her. He followed and we repeated this 3 times each. She used a garden hose like a douche washing out the cum. We did this many times before they moved. Were a poor family living in a borrowed home. Found out later the oldest son was doing his mom and sister as well.

Though many stories here are made up or fantasy, these are 100% true. The first girl and I dated for some time, I was 18 at the time and she 21 or 22. I dated her for one thing and I am sure she dated me for the same reason. She did tell me she stopped her brothers from screwing her, she had worry she might be pregnant. Fears it might have been mine, one of her two brothers, or her BF. oh, and the man next door. Seems he bribed her with rides to and from work. she repaid with oral and sex.
Being poor and not the sharpest girl around, No BC at all. She did what the other did, the sink sprayer up the pussy and let it flow. Thinking it washed out everything.

Hope this might release any guilt you may have from your own past.