I had sex with my sister and now we are having a full-on relationship

I’m 20 and she’s 17 — nearly 18. My dad walked out on my mum when I was one years old and we’ve not heard from him since. Then mum’s boyfriend moved in and my sister was born soon after, so we don’t have the same dad. My mum enjoys being with men and there was an endless succession of them throughout my childhood.