Pretty strong start. There's a lot to like here. Art style and character designs are good. Any voice acting is a win and this is fully voiced in English, though it's a bit dry and lifeless.
One thing I'd like to see going forward is for stronger dialog. It can be a bit simplistic and to the point. Also, I would let some of the intrigue and mystery linger a bit longer. It feels like we're insta-solving a lot of the misunderstandings and doubts (see spoiler).
I think the greater plot surrounding your sister would be better served by having all the other little mysteries maintained. It would give us more misdirection and red herrings to pursue while following the main quest. Everything gets settled too cleanly and promptly. We should be left in doubt more.