Kathleen Mary was the eldest of six children born to Ruby and Morris Logan. Four survived into adulthood. Michael and Alice are mentioned on Da's (Morris') death certificate and Aunty Tess knew about them but they were both likely to be full term stillborns and no records were kept. It's possible that they were born in the gap between John and James (1919 and 1923).
The records do show that Kath and Jack (John) were both born at 43 Garden Street, South Yarra which would have been the address of the old South Yarra Fire Station (or close by, if the family didn't have quarters at the actual station.)