Onii-Chan, Where's My Kiss? (there's four sisters, but you can only focus one sister at a time, and each has their own story)
I can't think of any other games that focus entirely on one sister, so here's a few that come close:
Kindread The Redburns (mother and sister with incest patch, I don't recall if you can ignore the mom and go sister only though)
Sistersitting (text based, but very good)