Aside from the eldest, Donald, born in October, 1930, the three younger daughters were born as follows:
Barbara (Barbie), 1944
Virginia, September 19, 1946
Susan, Jan 9, 1948
I've never found any evidence of adoptions or missing/deceased siblings, despite the unusual 13-14 year gap between Donald and Barbara.
The Martins moved into their house in 1932-33, but given the state of the country at the time, the Depression, I'd find it hardly surprising if Kenneth and Barbara Martin deliberately stalled having any more children for several years....with Barbara born in 1944, that made father Ken 40 and mother Barbara 34, which would be approaching the end of childbearing years in that era...so they probably decided by then, in 1944, that their situation had improved enough to make the best and have three more children in quick succession, by no means unusual then. Birth rates plummeted during the early to mid 1930s, and only began to rebound during WW2, then the Baby Boom starting c.1946.