If you are looking for the mother's siblings, then the 1930 and 1940 US Censuses are possibilities.
Obituaries of family members usually indicate children, siblings -- although often they indicate only the survivors. Her parents' obituaries might list their grandchildren, for example. ALL family members' obituaries should be sought, since even a maiden aunt's might list her nieces and nephews.
Some city directories would list family members if the children were older (say, high school or college.)
Often siblings attended the same schools, so if you find one's info you may find the others...