Sci-fi from late 80's about sibs modified for space life

[sketchpadnc]

I'm looking for help with the name of a book I read in the late 80's. The main characters were a brother and sister that discovered the often used "this could change everything" thing. Gene mods were a large part of the book as many characters were adapted to life in space... The brother had three arms but I dont recall the sisters.

I do recall that the brother played guitar. There was one point where he was playing a jam session and the lead singer was ripping up his throat trying to sing "half cord" or "off cord". That's about all I recall and I know its not much but any help will be appreciated. It's driving me crazy that I cant recall the name.