This started 5 years back. My fiancee's age was then was 28 and her sister's age 31. We got married the same year. The wife's name is Sagrika and her sister's name is Niharika.
This was 10 months before my marriage when my fiancee's parents had to go out of the city for a couple of weeks. Sagrika was staying with her sister Niharika at their home who had recently been divorced. I used to spend most of my evenings at her place giving company to both. In a sense, I was part of the family only long before marriage.
One evening Sagrika and I decided to have sex as we were supposed to be alone at her home unaware Niharika was in the bathroom getting ready. She saw us both naked in action, just came out of the bathroom, greeted us most strangely, and left the room asking us to continue.
To my fiancee also it was like nothing had happened. Didn't feel awkward at all. There were no secrets between the two sisters, her elder sister knew about our sexual relationship. Both sisters shared the same room. In monsoon season at times, I used to spend nights at her home because of roads getting flooded. Eventually started to spend most time at my fiancee's place. Her home also had a only single AC working at that time. So Niharika only insisted I sleep in their room.