Hello Denise
Oh my how I hope this will help you. My own grgrandmother was born in Patna in 1869. (I wonder if they knew each other

) Actually the birthplace of mine is recorded as Dalrymple but I know they were living in the miner's rows above Patna.
I'm betting that your Isabella was not born in 1866 but perhaps a bit later...but the IGI records show a different surname for what I believe to be her mother!!
ISABELLA MCILWAIN
Female
Birth: 09 FEB 1868 Dalrymple, Ayr, Scotland
Parents:
Father: ANDREW MCILWAIN Family
Mother: CATHERINE MACMURRON
--------------------------------------------------------------------------------
Source Information:
Batch No.:
C115881
These are the other births fromt he IGI to this couple:
4. EDWARD MCILWAIN - International Genealogical Index
Gender: Male Birth: 27 FEB 1866 Dalrymple, Ayr, Scotland
5. ISABELLA MCILWAIN - International Genealogical Index
Gender: Female Birth: 09 FEB 1868 Dalrymple, Ayr, Scotland
6. ANNIE CATHERINE MCILWAIN - International Genealogical Index
Gender: Female Birth: 11 JUN 1874 Kilwinning, Ayr, Scotland
7. MARGARET MCILWAIN - International Genealogical Index
Gender: Female Birth: 08 MAR 1872 Dalmellington, Ayr, Scotland
In 1881 there is a Catherine, Margaret, and John that might be from this same family
Dwelling: Maybole Row
Census Place: Monkton & Prestwick, Ayr, Scotland
Source: FHL Film 0203606 GRO Ref Volume 606 EnumDist 5 Page 17
Marr Age Sex Birthplace
John BANNERMAN M 59 M Ireland
Rel: Head
Occ: Muslin Weaver
Agnes B. BANNERMAN M 60 F Monkton, Ayr, Scotland
Rel: Wife
Isabella CARSON W 60 F Monkton, Ayr, Scotland
Rel: Board
Janet MC SKIMMING 14 F Kirkoswald, Ayr, Scotland
Rel: Board
Ellen QUIN 11 F Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
Archie GIBSON 11 M Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
John MC ILWAIN 11 M Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
Margaret MC ILWAIN 9 F Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
Catherine MC ILWAIN 7 F Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
John FULLON 8 M St Quivox, Ayr, Scotland
Rel: Board
Occ: Scholar
Annie SLOAN 7 F Dalmellington, Ayr, Scotland
Rel: Board
Occ: Scholar
Catherine MC KILLOP 4 F Maybole, Ayr, Scotland
Rel: Board
Agnes THOMSON 9 F Ayr, Ayr, Scotland
Rel: Board
Occ: Scholar
If it is them then perhaps both Andrew and Catherine were dead by then.
A lot of speculation but perhaps some clues that you could search out!!
Hope it is helpful
Best wishes
Jean