Hope this account sheds some light.
In the Napa Region of California, there lived four Watson boys: John (1830-1884), Ringrose (1833-1887), George (1822-1891) and Felthan (1827-1902). It turns out that George & Felthan were brothers, and John & Ringrose were brothers. John never married. When he died, he left most of his property to the children of George. John and Ringrose had a sister, Margaret Sommerville (1840-1925), who objected. She traveled to Napa, and sued the estate. The local newspapers covered the trial in detail. The implication was that George's wife convinced John through her wily ways to leave the property to her children. Here are some excerpts from the trial:
from "The Napa Register", 20 May 1887 (newspapers.com)
Mrs. Margaret E. Sommerville sworn - Was born in Ireland in the year 1837; my mother had ten children, and myself and Ring are the only ones living; I had eight brothers; John Watson whose will was admitted to probate was my brother; he came to this country from Ireland; think he was 19 then; I am married and my husband is now in Washington Territory; he has had no occupation since 1877; before that he was manager at the Royal Gun Powder Works in Ireland; he lost his position in '77; I have seven children living and four have died; I had seven children living with me in 1877; in January, 1880, all were living with me except my eldest son; when my husband lost his position we had no income; ....
From "Vallejo Evening News Chronicle", 20 May 1887 (newspapers.com)
[Margaret Sommerville testimony] After my mother's death in 1870 both John and Ring wrote to me sometimes...I had a brother Benjamin who died several years ago in Australia; think he died late in the seventies and he left a widow and several children; Thomas Watson, another brother, died either in Australia or New Zealand also some years ago and left a family.
from "Napa Register", 27 May 1887 (cdnc.ucr.edu)
Felton H. Watson. - Reside at Vallejo; have lived in Napa county since '55; Geo. Watson is my brother; John Watson's grandfather and my father were first cousins; I left Dublin in '52; Ring and John accompanied me and we three started to St. Louis, U.S. in a sailing vessel; John, George and myself then went to Soscol; I left sometime after for the mines but John remained in the employ of George; Geo. settled in Soscol in '55, and has lived there ever since; between John and George Watson and family there were very friendly relations;
from "Death certificates (Washington (State), 1907-1960" (familysearch.org)
https://www.familysearch.org/ark:/61903/1:1:HVWC-WWT2Margaret Everina Sommerville, a housewife, born 26 Aug 1840 in County Clair, IRE to Henry Benjamin b. IRE & Mary Ann Annesley b. IRE died 20 Nov 1925 near Centralia, Lewis, WA of chronic nephritis. Burial in Centralia WA. Informant Robert Sommerville.
Assuming Margaret Somerville's grandfather is Benjamin A Watson (as stated in an earlier post), then he would be a nephew to Felthan's grandfather, Felthan Watson (1744-1790) who married Anastasia Drew.