I didn't find anything about Mary, Alfred and Helen after the 1930 Norwich, New London, CT Census.
I'd look for her parents obituaries to see if she's mentioned in them.
Born: abt. 1882
Res: Norwich, New London, CT
Died: 30 July 1961, Norwich, New London, CT
Source: CT Death Index 1949-2012
Born: abt. 1885
Died: 23 June 1963, Norwich, New London, CT
Source: same as above
The Otis Library, www.otislibrarynorwich.org/new-page-3/
has Norwich newspapers 1773-present.
Also there is a list of their children and their death dates (except for Mary) at the Podurgiel/Kilbosa family tree, ancestry. Successful Searching!