A search for S* Gepner, narrowed down to children born +/- 5 years of 1930, turned up only one male, Szymon.
I have also researched his (likely) sister, Krajndla, and completed her profile with similar information.
We think Sura Mindla Gepner is the mother since she was born in 1900 and lived at Schmiede 8 4 and Buschilnie 3 5 with Szymon.
Krajndla Gepner may be a sister. She was born in 1928 and also lived at Schmiede 8 4 and Buschilnie 3 5.
Icek Gepner may be a brother. He was born in 1925 and lived at Schmiede 8 4 and Buschilnie 3 5.
Efrojm Ojzer Gepner may be the father. He was born in 1891 and lived at Schmiede 8 4.
We know Szymon was alive 7/1/1943 since the ghetto note Buschlinie 3 5 ABG 7.1.43 Rubens 9
I searched for everyone with the last name Gepner that lived at one or both of the same places as Szymon. I found his likely father (Efrojm/Ojzer, b. 1891), mother (Sura/Mindel, b. 1900), and possible siblings Abram (b. 1921), Chaja (b. 1923), Icek/Icchok (b. 1925), and Krajndla (b. 1927/1928.)
Both Krajndla and Sura Mindla/Mindel Sura were deported on September 10, 1942, on which date (according to Secondary Sources) there was a deportation to Chelmno. This is where they likely ended up.
Icek/Icchok and Szymon are both listed as moving to a new address-Rubens 9--on January 7, 1943. No other information is listed about their fate.
Efrojm/Ojzer died on July 1, 1941 of currently unknown causes.
I was able to put together pieces of Abram's fate from all the databases where his name showed up. He changed addresses and moved to Rembrand on August 13, 1941. I found that he was born in Litzmannstadt due to information found on a Mathausen inmate list. He appears to have survived Mathausen and the war, as he entered Ebensee Displaced Persons camp on March 9, 1945 and his name was also released on a Sh'arit Ha-pl'atah (survivors) list, which listed his last known location as Salzburg.
No information is known about Chaja's fate.