He asked where, and informed them they were all presumed to have died in WWII. They informed him one of his cousins was a near counterpart on the other side and provided a village name. Then they asked “would you talk to your parents and tell them to tell the neighbors it’s ok to talk to the G-Men”.
The FBI had apparently been going thru his tiny upstate hometown and scaring all the Polish/Czech/Slovak/Latvian/etc emigre neighbors who spoke little English.
Of course he didn’t tell them his sister wrote a letter to that village, re-established contact and then she, her husband and pre-teens (who only spoke English) flew over to Europe and took a train to visit. Thankfully everyone on the train distracted the security officials whenever they got near the kids.