@jespah,
I was confused about that too -- on re-reading I think the author (a girl) was with her cousin (also a girl) and the two girls met up with two guys. The girl cousin kissed the guy she was with. The girl author kissed the guy she was with.
oneya, if you think it's likely your mom will find out anyway, definitely better to tell her.
But you probably know best what her reaction would be (better than we would anyway). My own kid is almost 12 and if she told me she kissed a boy I wouldn't be upset -- I might be more upset if I felt the situation she'd gotten herself into in general was unsafe. (If the boys in question were much older, for example.)
But some moms might be upset.