Do you happen to know if his father naturalized as a US citizen when Sanders was born? If not, then he is yet another Article II ineligible Democrat POTUS candidate. He would then be a 14th amendment citizen at birth, NOT a natural born one.
No worries for Bernie though, since no one seems to want to ask the SCOTUS what does in fact constitute a natural born citizen. As Justice Clarence Thomas said so sardonically of that very question, “We are evading that one.” (Thomas’ testimony before a House appropriations panel discussing an increase in the Supreme Court’s budget in April of 2017. Thomas said that to Subcommittee Chairman Rep. Jose Serrano, D-N.Y.)
No, I have no further knowledge about his parents.