It was actually never said on the show. Maxwell's wife (Sara) died many years before the series started.
Maxwell Fry died in 1987.
Maxwell Maltz died in 1975.
Jenny Maxwell died on 1981-06-10.
Steamer Maxwell died on 1975-09-11.
Maxwell Munden died in 1973, in London, England, UK.
Actually, it was never said on the show.
Maxwell Maxwell-Gumbleton died in 1952.
James Clerk Maxwell died in die
Bochum.
Maxwell Reed died in 1974.
Atareta Maxwell died in 2007.
Donald Maxwell died in 1936.
Maxwell Kogon died in 1980.
Alexander Maxwell died in 1730.
Maxwell Fernie died in 1999.
Maxwell Bodenheim died in 1954.
Maxwell Hunter died in 2001.