Complex sentence: Regina returned to Vilna, but Max and Uriel stayed abroad, moving to New York in March 1940, where his wife and younger son Gabriel joined them during the brief period when Vilna was in independent Lithuania.
Split into simpler sentences: Regina returned to Vilnius, but Max and Uriel stayed abroad, moving to New York City in March 1940. His wife and younger son Gabriel joined them there during the brief period when Vilnius was in independent Lithuania.