I had a look at the deported files in Karta and lists.memo.ru just to see from which areas were deported Siedlecki families. You mentioned Brest is where your father came from. It would appear that the deported were mainly Warsaw born Siedleckis ( Sokalow and Dlugoszczow?), who lived in Bialystok - Brest area in Belarus.I cannot see a Jan, but there were Abram, Wladyslaw,Jozek, Jozef and Moisha.
I feel that perhaps concentrating your search on Bialystok area might give you some contact, particularly if you knew where your father lived-- I mean which church he was registered at birth. I am not familiar with place names near Warsaw.
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