I don’t know, we can only guess
The dark-haired girl laughed and said something in Italian to Sebastian, her voice questioning.
“No”, said Sebastian. “Lei e mia sorella”
“What did he say?” she asked. “In italian?”
“He said, ‘No she’s my sister’” said Jace. He did not say what the girl had asked to Sebastian
“Get over here. Sheldon’s had an idea.”
“Who’s Sheldon?”, said Isabelle.
Not officially but it’s been a while since the book came out so I hope there’s no problem with it