Where to look for those who are English but the child possibly born in France.

[Ashtone]

Sarah says she was born in Upavon in the 1851 census (which she only survived by a short time). From her stated age 59 her year of birth would be about 1792

Is this Aunt Sarah's baptism? No father mentioned.

Baptism date    28 Mar 1790
Place    Upavon, Wiltshire
First name(s)    Sarah
Last name    Maslin
Mother's first name(s)    Elizabeth
Mother's last name    Maslin

[Ashtone]

Bingo! Elizabeth Maslin's burial below.

Wiltshire Burials Index 1538-1990
First name(s)    Elizabeth
Last name    Maslen
Age    55
Birth year    1767
Burial date    21 Aug 1822
Place    Upavon
Residence    St Mary, Rotherhithe, Surrey  (where daughter Sarah married in 1822)

Here's Elizabeth's baptism:

Wiltshire Baptisms Index 1530-1917
First name(s)    Elizabeth
Last name    Maslen
Place    Upavon, Wiltshire
Baptism date    12 Apr 1767
Father's first name(s)    Richard
Father's last name    Maslen
Mother's first name(s)    Jane

Given the three items below, is there a strong chance that Louisa Elizabeth is the daughter of Richard Sissons and Sarah Maslin?

1.  "Aunt Sarah" married Richard Sissons in 1822 (in Surrey)
2.  Louisa's birth year is circa 1821 (birthplace is given as London)
3.  Louisa gives her daughter "Sissons" as a middle name

[Ashtone]

The marriage record on Ancestry is a bit of a mess. Spinster altered to widow. Her father is recorded as Richard Mortimer (butcher)...

Well, this is one for the books! There actually was a Richard Mortimer (butcher) residing in Upavo, Wiltshire. There's an 1804 Apprenticeship record mentioning his name:

"Robert HAMPTON, son of Edward (sawyer); apprenticed to Richard MORTIMER, butcher of Uphaven".

So it appears "Aunt Sarah" knew her father's name after all (even though no father's name is listed on her 1790 baptism record).

Here's another record, but looks like it may be for Richard's son.

Wiltshire Social & Institutional Records 1123-1968
Record type Petty Sessions
First name(s) Richard, Junr.
Last name Mortimer
Notes Butcher of Upavon
Date 28 Nov 1820
Place Upavon

So, does this mean that William Grace's father really was Thomas Chandler?

[Ashtone]

This could be James Chandler, witness at Louisa's 1847 wedding?

Wiltshire Social & Institutional Records 1123-1968

First name(s) James
Last name Chandler

Notes Of Devizes St Mary: Innkeeper & beerhouse keeper - The Three Crowns
Notes: Innkeeper & farrier of Week (Devizes)

[LizzieL]

So, does this mean that William Grace's father really was Thomas Chandler?

Amazing finds Ashtone!

William Grace (1851 census) says he was born in Everleigh Wilts abt 1781. I did find one to fit s/o John and Sarah.
But could Sarah have misunderstoof the "father" question and given the name of the Father of her previous (Chandler) husband?

[LizzieL]

Here's Elizabeth's baptism:

Wiltshire Baptisms Index 1530-1917
First name(s)    Elizabeth
Last name    Maslen
Place    Upavon, Wiltshire
Baptism date    12 Apr 1767
Father's first name(s)    Richard
Father's last name    Maslen
Mother's first name(s)    Jane

Image of the 1822 marriage on Ancestry
Witnesses were Richard and Jane Maslin (sic). All parties able to sign their names.

[LizzieL]

Richard and Jane (Elizabeth Maslen's parents) baptised a son, Richard in 1769 in Upavon. If he married a Jane, the witnesses could be Sarah's aunt and uncle rather than her grandparents.

[LizzieL]

Sarah's second marriage took place in 1835 to Richard Chandler (a widower of Pewsey) in St Pancras. Witnesses both had surname Hillier. One is Edward, can't read the other. Image of register on Ancestry

[Ashtone]

Haven't progressed on the search for Louisa's birth, but want to offer up the people below. No doubt the OP already has the details:

1.  1854 marriage in Islington for Louisa's "sister" Sarah Maslin Evett. I'm unable to view the marriage record, but I'm curious as to who she names as her father (and his occupation).

2.  1848 marriage in Yeovil for a Jane Maslin Evett. Is she another "sister"? What's her story?

3.  The OP references a "Mrs Fawkes of Carlton Chambers, Regent Street, London" in connection with sisters Louisa and Sarah. In the 1841 census there's a Joseph & Eliza Fawkes residing at Carlton Chambers with two teenagers. Joseph and Eliza are both servants.

I'm still no wiser about the Maslin sisters and Evett/Evitt connection. We need further clarification from the OP about the Evett people, whoever they are. Does the OP have the Evett's in any census reports?