So...
Claude told Alois that Sebastian was the one responsible for his brother's death, but... is it ever made clear why? I mean... he doesn't realise he wants Ciel's soul until well AFTER that point... so, for Claude to make that accusation about Sebastian beforehand... I mean... why?
My conclusion is that Sebastian slept with Claude's sister... or mother... or father... or, y'know, all of them, and Claude is just mega pissy about it.
Bassy is a Hell Whore. The boots pretty much confirm this.
Man... Claude... get over it. Your mother was a slut, anyway.