Yeah, her whole posture is wrong.
1) her elbow is suppose to be raised, should be parallel to the ground
2) hand position on the strings are wrong. Thumb should be on the left side of the strings and the rest of the fingers should be on top.
Other than that, the art is pretty and story interesting enough. Just need some tweaks on the plot since I feel like it's a bit rushed.
If you look at the other violin players in the chapter; like uke's two sisters and the guy in the page with Seme conducting Romeo and Juliet, you will see their thumbs are on the correct side. The female player's position is either a random mistake or the mangaka adding variety to the background. And I thought elbow position doesn't always need to be parallel to the ground. While some people play a piece, they change their their hand position according to which string they use at the moment, so that it would look dramatic.
I'm not a violin player but is she holding that thing right: http://www.mangago.me/read-manga/soredemo_ore_no_mono_ni_naru/mf/v01/c001/25/