Lenny Bruce, the brilliant New York Jewish comedian, insightfully declared that "in NY everyone becomes a Jew." Elaine is not Jewish by birth, she's Jewish by virtue of her environment. I should know, I'm a Catholic who has lived in NYC for 15 years. I travel around the country on business almost every week and everywhere I go people assume I'm Jewish. I'm also always being told by these non-NYers that I remind them of Elaine.
Non-Jewish New Yorkers are very "Jewish holiday aware" (alternate side of the street parking regulations are suspended and the traffic is very low, for example), Yiddish is the slang of the city, many of us have been to a bris and may even know a moyle (sp?) or two. We've also been know to attend affairs at Jewish social clubs.
Elaine is not Jewish, she's a classic New Yorker! And the longer she lives in NY, the more Jewish she becomes; hence, as the show has progressed, Elaine seems more Jewish.
The same argument holds true for George - George also seems Jewish. But with a name like Costanza, he's clearly not.
Valerie - Not Eric Miller