the hound is the valonqar. you can take that to the bank.
i think lots of people will do valonqary things, but hound is the one that makes sense to be described as "the little brother". tyrion and jaime are red herrings. grrm's playing irony by having cersei think physical stature designates who 'the little brother" is, when it's really gonna be a guy nobody would call little except he is still "the little brother" to the mountain.
cleganebowl will fucking happen.