Hi Judy45
Welcome to RootsChat

IGI have submitted entries for both Isabella's birth and also a brother John born 23 March 1845, like Isabella, also showing as born in New Cumnock.
Have you checked early census entries after the birth of Isabella to see what clues you can get?
There is this entry for example:
John Kerr 53, coal miner b. Sanquhar, Dumfries
James Kerr 23, son, coal miner b. Kirkcoonnel, Dumfries
Janet Kerr 20, daughter b. Kirkcoonnel, Dumfries
Elisabeth Kerr 14, daughter b. Durrisdeer, Dumfries
Thomas Kerr 10, son b. New Cumnock, AYR
Isabella Kerr 5, granddaughter b. New Cumnock, AYR
Jane Campbell 14, visitor
William Campbell 24, lodger
Address: Wellwood Row, Muirkirk, Ayrshire
Possible entry for Isabella living with her aunt Elizabeth in 1861:
Elizabeth Kerr 24, head, Muslin Sewer b. Durrisdeer, Dumfries
Isabella Kerr 13, niece, domestic servant b. New Cumnock
Address: Kirkgreen, Muirkirk
What occupation showed for Isabella's father John on her 1865 marriage entry? Did parents show as deceased at the time of her marriage? Do you know who were the witnesses to the 1865 marriage, any family names that you could work with?
Monica