Sebastian gets a letter from his mother one day, informing him that young Leophas has been making an absolute nuisance of himself and could Sebastian please take his dear little brother under his wing for a bit and show him the ways of the contracted demon?
Ciel thinks it's hilarious (after getting over the initial shock). Sebastian, with a mother and a brother! A brother who apparently ruffles the demonic feathers, disturbing that infuriating perpetual calm!
'Leo' arrives the very next morning, and Sebastian is going out of his mind within the hour. Ciel watches with interest as each of the butler's daily tasks are arrested, pranked, ruined, and interrupted in every way possible by this infernal child.
He constantly flaunts his wings in front of his brother, uses demonic magic on the tiniest things just because Sebastian can't, mocks Ciel (out of the earl's hearing range) and the contract, and messes up Sebastian's bedroom.
(To make matters worse, he's an absolute angel to the servants, who can't understand why Mr. Sebastian seems so uptight about havin' his little brother over fer a visit!)
Things come to a head when Leo releases Sebastian's beloved cats from the wardrobe and hides them in Ciel's study.
Ciel promptly has a meltdown and orders Sebastian in no uncertain terms, at the top of his voice, and using language most unbecoming to his station to get that brat out of my house this instant, this is an order! And take those cats with you!
Sebastian has never been happier to obey an order.