This document shows Sonia (a.k.a. Sophie) Posner (Esther's mother) returning to New York from Amsterdam.
Here's a copy of the (modern) transcription copied from ancestry.ca
New York State, Passenger and Crew Lists, 1917-1967
Name: Sophie Posner
Nationality: USC
Arrival Date: 12 Oct 1961
Arrival Place: New York, New York, USA
Airline: KL
Flight Number: 621
The address in Flushing shows this is the right Sophie Posner.