I believe it was for his brother-in-law, a violist, that Sibelius wrote a rondo for viola and piano. It really is a great piece, does anyone know more about it or heard it?

I have skimmed quickly through Sibelius' catalogue of works in Andrew B's book, but could not find a Rondo for viola and piano.

There is however a Rondeau Romantique for violin and piano, Op. 116 No. 3. (nos. 1+2 ar Scene de danse* and Danse caracteristique).

Maybe I skimmed too quickly... Andrew B will tell us more.
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* I wonder... would Scene de danse in Finnish be "Tanssikohtaus"? Sounds like a distant relative of mine.

Yes, I'm afraid you skimmed too quickly - see pages 84 & 393

The Rondo in D minor for viola and piano (JS 162) It is a shortish piece - about 4 minutes - from 1893 and was indeed dedicated to (and used as a Christmas present for) Arvid Järnefelt. Masterpiece? - No, I wouldn't go that far; but it's an interesting piece, more serious in tone than most of the 'souvenirs' written for family and friends. Partly that's because it is later than most of the so-called 'early' chamber muic, and partly it's because it reuses a theme from the finale of the Piano Quintet in G minor from 1890. That finale had not been performed and, with no prospect of a performance on the horizon, Sibelius was plainly keen to salvage what he saw as a valuable musical idea.

The Rondo has recently (2002) been published by Fennica Gehrman and is available on disc in a fine performance by Nobuko Imai (BIS-CD-829).

Though while it isn't a masterpiece, I think it's a pretty good addition to viola repertoire... Better than many viola pieces, I think.