Source: her marriage record at EA.
I think she may be the daughter of Andrew partly because of the names of her children and partly because her husband was PAD for an Anne Boyd, who I think is Andrew's granddaughter. Her Civil Marriage record confirms this. A Mary Anne Williamson was one of the witnesses, which is interesting as her deceased sister-in-law was called Williamson.
Also the births of her children:
SARA JANE SMITH
Birth:
05 OCT 1864 0405, Aghadowey, Londonderry, Ireland
Christening:
Parents:
Father: HENRY SMITH
Family
Mother: NANCY BOYD
Record Type Civil Birth
Date of Birth 25 Mar 1866
Name Andrew SMYTH
Gender M
Fathers Name Henry SMYTH
Mothers Maiden Name Nancy BOYD
Civil District Coleraine
Sub District Aghadowey
County Londonderry
Record Type Civil Birth
Date of Birth 12 Dec 1867
Name Henry SMYTH
Gender M
Fathers Name Henry SMYTH
Mothers Maiden Name Nancy BOYD
Civil District Coleraine
Sub District Aghadowey
County Londonderry
SMYTH, Andrew Birth
Gender: Male
Birth Date: 1 Feb 1870 Birthplace: 539, Aughadowey, Derry, Ire
Recorded in: Londonderry, Ireland
Collection: Civil Registration
Father: Henry SMYTH
Mother: Nancy BOYD
Source: FHL Film 101200 Dates: 1870 - 1870
Sarah Jane SmithEvent(s):
Birth:
13 JUN 1878 Aghadowey, Londonderry, Ireland
Christening:
Death:
Burial:
Parents:
Father: Henry Smith
Family
Mother: Nancy Boyd Smith