Her brother being first chair, if this is in a high school orchestra and he's only 9th grade, he sounds quite accomplished. This lovely Baroque concerto movement by Telemann may be within his grasp, and the piano accompaniment doesn't sound like something an intermediate piano student couldn't handle.https://youtu.be/2-B3Hif5Gxc
Here's a delicious, slower movement from another Baroque master, Arcangelo Corelli:https://youtu.be/hKJFutwe6iQ
This is a gorgeous piece from the Romantic Period, Jean-Baptiste Accolay's well-known, wonderful one-movement violin concerto for students (played here with a version for violin and piano):https://youtu.be/DSJvUxgvLvg
I'll add one more to this list. The piano accompaniment is more difficult than the others I posted above, and ensemble issues are more tricky between the players, but in case you're looking for something a bit more challenging for your student than the preceding piano parts offer, this may fit the bill, if she is more toward the upper intermediate levels and has played in ensemble with her brother before. These are Schumann's Three Romances, Op. 94, truly lovely works:https://youtu.be/x0nlXnkhBPQ
Good luck! Sounds like a fun venture.