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Looks like you've done some research, so forgive me if I make some suggestions you've already tried.
If you think there's a possibility Wm & Nancy stayed in Anderson Co, TX, and died there, have you checked for probate, land, tax records, and guardianship records? If there was no will, there still should have been a probate. If there was any land, all heirs would have to sign off, if it was sold. If the children were minors, a guardian should have been appointed by the court, and the guardian would sign for the children. And, tax records were every year, if Wm disappeared off the tax roll and his wife was on the next one, she was a widow. If Wm disappeared and his wife didn't appear, they either moved or died. There's always the possibility that Wm died, and Nancy had to "farm out" some of the children. By that time period Nancy could have applied to be made guardian of the children. Yes, even if the mother lived, females didn't count for much. A guardian had to report to the court any sale of land or estate, to protect the minor children's interest.
The half sisters, did they have the same mother as your grandfather, or same father? Did Joseph Barrington raise any of the other children from the family, or just your grandfather? Sounds sad, but interesting.