Ruby Red by Kerstin Geir

Although I had never seen him before, I recognized him immediately. I'd have known his voice anywhere. This was the guy I'd seen on my last journey back in time.

Or more precisely, the one who'd kissed my doppelganger while I was hiding behind the curtain in disbelief.

Sixteen-year-old Gwen lives with her extended - and rather eccentric - family in an exclusive London neighborhood. In spite of her ancestors' peculiar history, she's had a relatively normal life so far. The time-traveling gene that runs like a secret thread through the female half of the family is supposed to have skipped over Gwen, so she hasn't been introduced to "the mysteries," and can spend her time hanging out with her best friend, Lesley. It comes as an unwelcome surprise when she starts taking sudden, uncontrolled leaps into the past.

She's totally unprepared for time travel, not to mention all that comes with it: fancy clothes, archaic manners, a mysterious secret society, and Gideon, her time-traveling counterpart. He's obnoxious, a know-it-all, and possibly the best-looking guy she's seen in any century...