Looks like there is a Frank potential for confusion here. In the 1900 Johnson County, Georgia census (Wrightsville District, SD 11, ED 53, sheet 19B, dwelling 343, family 362) I find:
Amerson, Frank, head, 26 WM Wd Jan 1874 Farm Laborer
", Lillie, daughter, 6 WF, Apr 1894
", Thomas L., son, 4 WM, Feb 1896
", Dollie, daughter, 2 WF, Mar 1898
", Martha, mother, 65 WF Wd, Jan 1835
Frank is known to be the son of Appleton G. Amerson and Martha Hattaway from the 1880 Washington County, Georgia census (household and family 196). Note that Frank's birthyear is estimated at 1871 from the 1880 census, and that his father is quite alive in Jun 1900 and living with Frank's sister nearby. We might assume Martha is living with Frank to help take care of the children but that Wd is a mystery.
L. Frank Amerson is buried in New Hope Church Cemetery beside his second wife, Cassie, who d. 19 Aug 1932. His parents are buried in the same section of that cemetery. Frank and Cassie are in the 1920 Emanuel County, Georgia census (58th [GM] District, SD 12, ED 27, sheet 10B, dwelling 83, family 86). The copy I have is blurred so some info is missing:
Amerson, L F, head, ?? WM Farmer
", Cassie, wife, 34 WF
", Lorenzo, son, 15 WM
", Burfort, son, 13 WM
", Martha, daughter, 11 WF
", Leroy, son, 9 WM
", ????, daughter, 8? WF
", Robert A., son, ? WM
", N??, daughter, ? WF
", Dollie, daughter, 20 WF
Note that Dollie Amerson is in both households.
None of this is inconsistent with a 3rd marriage, but either your listed children Johnnie, Dorothy and Jack are Cassie's children or Frank and Cassie divorced before Frank and Ruby married. It would seem odd for Cassie to be buried next to Frank in that case.
I will pick up the 1910 and 1930 census entries next week at NARA and post them here for our mutual enjoyment.
As for Frank's siblings, his mother was a Hattaway, and like Frank she follows the Hattaway rule: the more the better. She had 16 children according to the 1900 census. I know names for 10, one of whom died in childhood. As far as maternal aunts and uncles, I have names for 13 of those. (This is a roundabout way of saying it's too much to type in at 3 a.m.)
I'm researching Martha Hattaway Amerson's family in the 20th century censuses now and will get FGSs together when I'm through with that.