That's Lesley Nelson's site, incidentally:  Francis J. Child Ballads.  Here is a link to the main file:  Brother's Revenge

The Cruel Brother was Child's Number #11 in his definitive English and Scottish Popular Ballads of 1884-98.   Strictly speaking, the Cornish lyric is the translation, and was probably made relatively recently from the English text; most extant Cornish language song texts are 20th century translations from older English language songs.  That's not to say that there may not have been genuine traditional Cornish language versions at some point, just that, if they existed, they mostly didn't get written down and were lost.  Does Stivell name his source for texts and tune?

Malcolm