Given the clue about Blanche being a sister to your father I went searching on the 1910 and 1930 censuses. I had always thought Madge/Maggie's (Margaret Northrup/Northrop) first husband died before 1920. I am guessing they divorced. I see a Gordon Severet born 1930 to Elsie and George Severet of Oakland, Alameda Co., CA.
Any information you have on Severet half-sisters Blanche and Pearl and on their children is much appreciated!
Long Beach, CA