From my story Know my story a short snippet of how the twins got their names.
OMAKE
Real Ciel has actual name
Ciel 'HEY'
Cielle "HEY"
"Hey cielle" mey-rin's voice rang out at the bottom of the stairs next to her master. "If his names really Ciel what's your real name." Cielle gave her a deadpanned look "it's Cielle spelled C-i-e-l-l-e." "Sooo" Bard spoke up cigarette lit in mouth with his eyes closed in question. "You guys have the same name." Tanaka spoke up this time "Mrs. Phantomhive wasn't really one to come up with names neither was the Earl so it resulted in the birth of one Ciel and Cielle phantomhive." Finny had a giddy tone as he asked "what if they had been born girls." "I'm afraid the results would have been worse finny there would be two twin girls running around named cileste and cilleste." "They weren't creative at all' Mey-rin screamed out. Both Ciel and Cielle turned to each other earlier feud gone as the servants began to rant about their names. Cielle looked at Ciel and asked "hey you wanna play crochet." Ciel shrugged and they both walked off hand in hand as brothers. 'Hey Cielle.' "Yea?" 'I'm still taller than you.' "Bastard." 'Right back at ya.' Sebastian looked back at his master eyes widened from shock.