I had a similar experience with two sisters. I met the first sister on SA about 2 years ago. She was 23, a recent college grad, had just moved to VA, and was working as an intern in a local law firm. We hit it off immediately and began meeting weekly for lunch or dinner and "dessert" behind closed doors. Her company was great and the sex was incredible.
I met the second sister on SA six months later. She was 25, had been living in VA almost 2 years, and worked as a model at a local agency. We also hit it off immediately, and mostly met for just drinks and sex. She was a dynamo in bed.
I kept both in my rotation, and after a year, was surprised to discover they were sisters. Although they discovered me on SA under the same name, they swear they never compared notes or swapped stories. They did not live together, but once when I was at the second sister's apartment drinking as our prelude to sex, the first sister stopped by to pick up some clothes.
We all three spent that night together, and several other nights. We also continued to date separately until The first sister moved back home to MI, and the second sister moved to GA.
It was a mind blowing experience we all enjoyed.