Is this like... Sister NTR? You don't have a gf? I've just started playing and saw a sister, that's why I'm asking
It's as the guy said above, you either save the girls and get to choose one for yourself or lose them to either barbarians or the king. And your little sister is also one of the girls that can get taken, but she is one of many.
This is one of those games that is mostly for those who want a challenge to save the girls.
However, I do highly rate it right up to the point of taking the capital city. A great netorare scene hits if you didn't pick the correct dialog choices. After that the gameplay was mea for me.(That's when the main "keep the girl unfucked" gameplay starts) But that long build-up to that scene made the game worth it for me, in fact its one of my all time to because of it.
I only did the main Leane path, you can pick a few different starting areas, which determine your initial party. Not sure if there is a good NTR that smacks you unexpected like with the Leane path, maybe someone else can say? But if you take the leane path and just play till the capital is retaken, I think its worth a go. And stop there if you don't like the combat. I did get a hard crash when I retook the capital, but there is a patch in this thread somewhere. You only get it if you dialog choices unlock a specific scene I think.