Jules van Os, the young man who was living with the Velt family, was not a stranger.
Rica Velt's sister Catharina married a man named Hartog Cohen. Hartog Cohen's sister, Mietje Cohen, is the mother of Jules van Os. So, to Rica and Marcus Velt, Jules van Os was their brother-in-law's nephew, which is presumably how he ended up living with them.
Person is in the National Database of Victims of Persecution