Tracking Blossom and Meta is harder since we don't have married names for them, but the Rabinowitz boys should be trackable. I checked Julius and found a 1920 census listing Anna as spouse, and three children Seymour, Abner and Judith. Walter's history mentions that Julius had a daughter Judith who was friendly with Stan's sister Joan, and who was about Joan's age. Sure enough, Judith Rabinowitz is one year old in 1920.
Walter's history also has Julius working for the New York City school board as a truant officer, but the census records show his occupation as bookkeeper (1910) and clerk-dry goods (1920). Perhaps it was one of the other brothers, Henry, Seymour or Abe, who worked in the schools. Like his younger brother Walter, Julius registered for the draft in 1918 even though he was then 41 years old.
Checking the other brothers, there are several plausible Henry Rabinowitzes, none a truant officer. I have an Abraham Rabinowitz who matches Walter's description as a men's clothing cutter. In 1920, he lives in the Bronx with wife Celia and children Yetta and Isadore. For Seymour, I have a 1917 draft registration card listing him as an unemployed bookkeeper who is blind in one eye, a 1930 census record showing him living alone in the Bronx and working as a--aha, there it is--a truant officer in the public schools, and a New York death notice recorded in February 1963.