The Rabbi of Międzyrzec Podlaski, Poland ( Yiddish -Mezritch), during the C19 was one of the Charlap dynasty. A JRI-Poland search reveals a large number of BMD records for the name ‘CHARLAP’, of whom 60 are in Siedlce Gubernia. There is an interesting Miedzyrzec Marriage Record, 1881 M #46, between Moszka CHARLAP and Sura Rywka CHARLAP , clearly some sort of cousin marriage, which might be worth following up. The scanned image of the original record, which is in the Lublin branch of the Polish State Archives, can be viewed online.
It would be a good way forward to try to relate the family line that you have by word-of-mouth evidence with records available on JRI-Poland.
Researching` KUPERSZTEIN/KUPERSZTEJN, ROZEN, ZYLBERKRANC, from Międzyrzec Podlaski; PITERMAN, PRZEKUPNIK from Biała Podlaska ; KUPERSZTEJN from Pratulin, Janow Podlaski, Włodawa, Terespol; COOPERSTEIN , PETERMAN, from Cardiff, UK