Hi Heather
Welcome to RootsChat  

I can't help you with with details outside of what is available on line for searches unfortunately.  Hopefully someone may recognise the names from their own family lines.
In respect of what is available on line, this looks a possible entry for your father's grandparents' marriage:
1898   JACK   DAVID   COWIE   ANNIE   MACDUFF   /BANFF   155/A2 0019
There is also this entry on the 1901 census that might relate to them:
David Jack 29, fisherman, b. Avoch
Ann Jack 27, b. Peterhead Aberdeenshire
Margaret Jack    11 mo, b. Avoch
Address: 26 High Street, Avoch
The marriage cert which is available to view and download on Scotlands People (pay to view but v. reasonable) will confirm the names of David and Annie's parents. There are two possible entries on IGI at family search which could relate to their births:
DAVID JACK  Birth:  14 SEP 1871  Avoch, Ross And Cromarty, Scotland
Parents:DAVID JACK and MARGARET JACK
ANN COWIE  Birth:  27 JUL 1874  Peterhead, Aberdeen, Scotland
Parents:ALEXANDER COWIE and JANE MORRISON
Regards.
Monica  
