They had sex that is disgusting
Toby and Jenna are step siblings they have no blood relations
I also think Jason may have a twin sister!
How do we know they had sex? We never see them as a couple, they just say they were. Cece was making out with a guy (not Jason) at the bar near Hollis in Ali's flashback, and supposedly her and Jason were together then. It's a cover story, because no one knows they are twins. It seems Ali and Cece spent more time together than Cece and Jason. It makes perfect sense.