29 [M4A] I live with my girlfriend and her sister, and I have a sexual relationship with both

I own my own house. A couple of years ago, my girlfriend moved in with me. Six months ago, my girlfriend's younger sister moved in with us - she is at university in my city, but is very shy, hadn't really made friends and was lonely. Shortly after, my girlfriend told me that she knew I was attracted to her sister, that her sister was attracted to me, and that she wouldn't mind if I slept with her sister too. From then on, I've been sleeping with the sister, as well as with my girlfriend.

Message me if you want to know about my situation with them