This happened when we were in highschool.I am a girl.My brother was 15,I was 16 and my stepsister was 18.Me and my stepsister used to peck each other on lips while our parents were gone and it escalted to us making out.It took him a while but my brother caught us making out and my stepsister because she didnt want him telling our parents ended up making out with him and for a while me and my brother used to make out with our stepsister.I dont know if my brother and stepsister had s** but they were a lot closer after that.