When I was 15 I lost my virginity to my girlfriend who was half a year older than me. She had lost hers a year before to another guy at our school who she dumped to be with me. My mom didn't want me having sex let alone dating until I was 18. My girlfriend had a relatively similar situation with her dad.
Our parents thought we were just friends and through our "friendship" it brought my mom and her dad together. My mom was divorced and her dad lost his wife to a car accident when my girlfriend was just a toddler. They married when I was 17 and my girlfriend became my step-sister and I moved in to their house.
I ended up sharing a room with her until we went to college at the same school. We quickly moved out of the dorms and got an apartment together.
The first month our families joined we decided to no longer date or have sex, but the temptation was too strong and we had sex as often as we could with a few close calls to almost being caught by our parents. We are still dating and having sex today. A few of our friends know, but we continue to keep it secret from our parents.