I'm very close to my dsis, she's four years older than me (27, I'm 23). She has settled down in the past couple of years and moved in with a really lovely guy, I like him a lot.
I'm not coupled up at the moment so I'm not sure if I'm being a bit possessive 
But during all of our last few meet-ups (which have mainly been at weekends as I live far away), seeing her involves seeing them both as a couple. Eg we'll meet in town for a meal/drinks or occasionally go back to their house.
I don't actively mind as he's funny and nice and great company. I also talk to her on the phone at least once a week so we get to have personal chats then. I just feel it changes the dynamics slightly always having him there.
Aib for finding it kind of sad that I'll never get to see dsis alone?!