Viability: In broad terms, the game is viable. I was never confused with the options given, and I was able to role play my stance towards this energetic (or annoying) sister character. It is unfortunate that we really don't get all the way to that grand party at the end. I think that would have been a great setting give players even more meaningful choices. Likewise, there are some passages that lead to dead-ends, which diminishes player choice. Simply speaking, the basic ingredients of viability are here, but the game needs to be "finished" to really pay off mechanically, in my opinion.
Presentation: The writing does a good job establishing the emotional stakes of the story, and the styling is promising even in this minimal (unfinished?) state. Obviously, what this game needs is some imagery! There are many choices: from drawing the characters, backgrounds, or simply adding abstract backgrounds that further emphasize the emotional stakes. I believe that presentation is one of the biggest opportunities for improvement.
Theme: This is the strongest aspect of your game, and the most promising, if you decide to continue with the story. The relationship between the two characters feel well-worn, and the options you gave us allow us to role-play, as I said earlier. What makes the theme strong, above all, is the possibility of revisiting that past memory of growing up with the sister. To have a back story complicate the choices in the middle of the plot is inspiring! As I said in viability, however, I feel that the central moral questions in the story are not resolved by the end of experience. We need to go to that party!
Engagement: Like many interactive narrative works, I enjoyed exploring the multiple choices. It is unfortunate that we have dead ends and an incomplete plot at this stage. This story is very close to deliver something very arresting. Consider finish it in your spare time, or for your final project.