This explains the situation a bit better. Correct this if it is wrong...
To summerize: Like in the rest of Europe, the genocide of Jews in Serbia was primarily of Nazi Origin. (this part I already knew). The other groups that helped the Nazis in the Balkans, were primarily the Croats who were Catholic with aid from certain Muslims in Bosnia, but not all Bosnians. If there were any Serbians who sympathized with the Nazis, they were a tiny amount as the majority were sympathetic and loyal to the allies.
Would you say that is accurate?
Therefore, Israel was likely the destination of many pre-WWII Serbian Jews. If both these highlighted families went to Israel, they were likely following a trend.
It also could be that after the Nazis, there was little hope in the life in the Balkans for them to return to and the Jews felt that they had a better chance of a successful life in Israel.
Wasn't Israel officially recognized by the U.S. in 1948?
So, besides Serbia being poor after the war, many Jews wanted to live in Israel as a new officially recognized country. It held more promise.
But what I was pointing to, is that if you or others wanted to delve into serious research of Serbia's pre-war Jews, that Israel looks a good place to start as some Serbian Jews were documented as going there. I'd like to know myself, as things seem generally murky - that's why you need specific people/stories to get good details, and many of them. Two families is not enough to go on, you'd have to find more before you get a fair picture. Maybe there are many in Israel who have/had a grandfather or grandmother from Serbia or the rest of Yugoslavia.
Wouldn't Israel have the records somewhere on where people came from as they arrived in the late 1940's?
You could also query people living there (Israeli newsgroups?) on how to find this information.