If asked, Stefan would confirm that Damon was indeed a relative. His older brother, in fact. It's a good thing Stefan's unaware of Lex knowing Damon, else he would be plagued with hoping the man didn't ask about him. He and Damon weren't exactly seeing eye to eye at the moment to the point where Stefan hadn't been at the house in weeks.

As for the remark about everyone having a double... Lex, you don't know how right you are. There's a set of doppelgängers here masquerading as a pair of twins.

"I'm seventeen," Stefan answers, half-truth, half-lie. "Pretty sure that'd still be illegal, here. I don't even know how to play poker right, so..."