The MIs for her sisters state that they were the second and third daughters of Sir Thos P, so she must have been the first dau.

Her sisters were born c. 1691 and her brother Edward is thought to have been born in 1690, so this puts her before him.  Let’s say she was born in c.1688, halfway between her first and second brothers, Thomas and Edward?
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Martin P-L writes:

married Thos. Carthew of Beenham, Berks.  They had four children.

TFPL, Oct 2009: Vivian shows seven children in his Cornwall Visitation:

Anna, m. John Norris of Wychingham, Suffolk
Sarah, m. Henry Losson
Maria, m. Stephen Gardiner of Norwich, Barrister-at-Law
Elizabeth, d. unm. 1766
North, d. 1716,
Thomas, d. 1720
Edward, d. 1720.

And TFPL, Feb 2021: A thorough search of the baptisms has found five of the above seven, with Sarah and Thomas missing.
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In the parish register for Church Pulverbatch, Shropshire there is this entry:

  “Sarah daughter of Thomas & Sarah Powis (or Porvis?) baptised March ye 30 1688”

But it omits that Thomas was a knight...
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In the parish register for St Giles in the Fields, High St, Holburn, London there is this entry:

“November 1688           Christenings
. . . .
27 Sarah of Sr Thomas Powis Knt his Witness(?)
   Thomas Gen?? and Sarah Dvj born this day”
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Sarah is thought to have had three sons and four daughters.  I have already found that there were two separate christenings of daughters called Mary, increasing that number.

And in ‘Genealogical Notes’ by George Lockhart Rives, printed New York in 1914, is written:

“The Second daughter of Mr Carthew was the wife of John Norris—and the third eloped with and married a Coachman of her father’s, named Lossom, who treated her so ill that she died of a broken heart, leaving a son and several daughters, who were principally supported by their relations on the mother’s side, but who turn’d out ill and have long since left yo their fate.”

I have yet to find any record of this third (or maybe fourth) daughter.
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Also in Vivian’s Visitation of Cornwall, p. 574, it says she was the first wife and that her husband died in 1742.  So she must have died before 1742.  And the second wife had a son, Thomas, who was born on 4 Aug 1732, so she must have died before that.   c.bef.1730 sounds valid.
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`FindaGrave website reports that she died in 1727 and is buried in St Margaret Churchyard, Witton, Norfolk.  It also gives her birth as 1793, which does not accord with her christening five years before that.
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From their licence to marry of 26th May 1710 (year is unclear):
Thomas Carthew of the Inner Temple London and aged 23 and a batchelor and alleged to marry with Sarah Powys, the daughter of Sir Thomas Powys of the Parish of St Giles in the Fields ... Middx ... with the consent of her father to be married in the parish church of St Albans Woodford (?), London.

Signed by T Carthew.
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Wikipedia says it was a St Alban church in Wood Street, London and was wrecked, bar the tower, in WW2.
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At Last: The London Metropolitan Archives have found the problem and taken me to the marriage register page.  Her name had been transcribed as Povis, correctly to my eyes and the records reads as:

[City of London,  St Alban, Wood Street,  1662-1786]

“Weddings in ye Parish Church of St Alban Woodstreet 1709”

. . . .

“May … 28 1710 Married Mr Thomas Carthew& Mrs Sarah Povis, by Licence In St Alban Woodstreet Church by Mr Mouls Lecturer.”
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