As for the 1926 trip, I found a Dorothy Forster on the SS President Wilson from Manila to San Francisco 25 Aug. 1926. It says she was born Fortuna, California on 27 May 1911 and was living at 1630 Lincoln Ave., Alameda. No other Forsters appear to be on the ship. I don't think she matches up with the info above.
I also found the family on a passenger list for the SS Montlaurier from Liverpool via Greenock to Quebec, arriving 1 Sep. 1923:
Forster, Arthur B., age 40, male, married, naturalized at US District Court of So. California at Los Angeles on Sept. 7, 1918 (last number hard to read), address in US - Route 1, Box 59, Lankershim, Calif.
Forster, Alice Aubrey, wife, age 40, female, married, naturalized by husband's papers, same address in US
Forster, Dorothy A., child, age 9, female, single, born Jan. 22, 1914 in Los Angeles, same address in US
I just found Arthur's petition for naturalization, which gives Alice's date of birth as 19 Jan. 1874 in Llandoff, Wales.