Nice one, glad I thought of the brothers in question so quickly! (Didn't bother working out which would be CAIN and which ABEL, just jotted in SETH and went from there.)
Instrument that represents the duck in Peter and the Wolf
6A
Less-famous brother of 1A and 2D
7A
Jai ___, basque sport whose name means "merry festival"
Verticales
1D
La ___ Nostra
2D
Brother of 1A and 6A
3D
Ι as in Ionia?
4D
Old US soft drink whose company was renamed to the Royal Crown Company, then became a brand of Dr Pepper Snapple Group, which is now known as Keurig Dr Pepper