Also the Patricia Jaeger “…with a Friend” books (Irish, Christmas, Hymns) are nice. They’re for harp and up to four other instruments (come with string quartet parts, but they work fine with just harp and treble I).
Dear Julia, There is a very nice Suite for harp and oboe (or violin) by Bernard Andrés. The title is Les Algues (in english The algae). 7 short pieces without pedals changes. You can aldo try the Zerbina for violin and harp by same composer.
Pergolesi’s Siciliano and Bach’s Sicilano from sonata are very convenient for Christmas occasions. I hope this helps you.
Daniel Burton has a nice book with three Christmas duets, called Christmas Carols…it’s an orange-ish book which includes Still, still, still, Angels we have Heard on High, and What Child is this. The harp part is mostly arpeggios. It’s easy to simplify or embellish, as well. The violin part is fairly easy – your brother may need to know how to play in Third position for it to be able to work.
Also, look into the “Familiar Hymns with a Friend” book – they are easy harp arrangements with treble clef instruments to play with. That would work.
Also, even though there aren’t many harp/violin pieces, there are many harp/flute, which work just as well – just make sure the flute part doesn’t go up too high or have excessive runs – if they do, you could omit them.
Also, if you want to go really simple, if you have Sylvia Woods’ Hymns and Wedding Music, 50 Christmas Carols, etc., you could play those with your brother and have him play the melody.