I have no idea if this is related to the book this girl's parents read, but from Googling it seems that Ketrin is a form of
Katherine sometimes used in Serbia. It's unclear whether or not this is actually a form really used in Serbia, or if it's just a way Serbians traditionally used to represent "western" forms of
Katherine, but I have found several websites obviously using this form to respell the names of American actresses.
Katharine Hepburn seems to become "Ketrin Hepburn" in Serbian:
http://www.voanews.com/Serbian/archive/2003-06/a-2003-06-30-1-1.cfm
and
Catherine Zeta Jones becomes "Ketrin
Zeta Dzons" in Serbian:
http://www.krstarica.com/lat/magazin/film_muzika/index.php?clanak=jos_dece&arhiva=10-2001