Looked for her name just incase she was named after someone in the family that search in 1881 yielded these 2 matches: If not then the above is the index for all named Sim living in Merthyr in 1881:
Dwelling: 8 Bell Row
Census Place: Merthyr Tydfil, Glamorgan, Wales
Source: FHL Film 1342278 PRO Ref RG11 Piece 5310 Folio 60 Page 15
Marr Age Sex Birthplace
Wm. SIMS M 50 M Llansamlet, Glamorgan, Wales
Rel: Head
Occ: Colly Pumpg Eng Tender
Catherine SIMS M 46 F Abernant Merthyr Tydfil, Glamorgan, Wales
Rel: Wife
Mary A. WILLIAMS M 29 F Abernant Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
David SIMS M 21 M Merthyr Tydfil, Glamorgan, Wales
Rel: Son
Occ: Loco Fireman
Cath. SIMS U 18 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
Occ: Labourer Top Of Colliery
Margt. SIMS U 16 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
Occ: Labourer Brick Yard
Eliz. SIMS U 13 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
Occ: Scholar
William SIMS 11 M Merthyr Tydfil, Glamorgan, Wales
Rel: Son
Occ: Scholar
John SIMS 9 M Merthyr Tydfil, Glamorgan, Wales
Rel: Son
Occ: Scholar
Gwen SIMS 6 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
Occ: Scholar
Thos. WILLIAMS 10 M Merthyr Tydfil, Glamorgan, Wales
Rel: Grandson
Occ: Scholar
Wm. WILLIAMS 3 M Boston, America
Rel: Grandson
Sarah Ann SIMS M 22 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur In Law
Mary Ann SIMS 2 F
Rel: Grand Daur
Dwelling: Woodland House
Census Place: Merthyr Tydfil, Glamorgan, Wales
Source: FHL Film 1342278 PRO Ref RG11 Piece 5310 Folio 127 Page 6
Marr Age Sex Birthplace
Rachel EVANS M 47 F Merthyr Tydfil, Glamorgan, Wales
Rel: Head
Occ: Wife Of William Evans (In America)
Williams EVANS U 20 M Merthyr Tydfil, Glamorgan, Wales
Rel: Son
Occ: Colliery Brakesman
Tabitha EVANS 12 F Merthyr Tydfil, Glamorgan, Wales
Rel: Daur
Occ: Scholar
William DAVIES U 23 M Llantrisant, Glamorgan, Wales
Rel: Lodger
Occ: Assistant Road Surveyor
Thomas WALTERS M 32 M Mountain Ash, Glamorgan, Wales
Rel: Lodger
Occ: Quarryman
Simon SIMS M 21 M Tredegar
Rel: Son In Law
Occ: Clerk Of Building Works
Mary SIMS M 18 F Eglwisilan, Glamorgan, Wales
Rel: Daur