Hi there,
Since Susan has kindly provided Joshua's birth and the 1851 census details, here's a list of Joshua's siblings, in birth order;
George, christened 22nd March 1842 Tarves
Mary Ann, christened 15th March 1848 Tarves
Isabella, christened 8th March 1850 Tarves
William, christened 26th October 1852
John, christened 18th August 1854 Tarves
Ann, born 14th November 1856 Tarves
James, born 31st December 1858 Tarves
Douglas, born 2nd September 1861 Tarves
Gordon, born 4th November 1863 Methlick
Quite a big family! I can't find a marriage for William and Ann, but there may be a date for it on one of their later children's birth certificates. All certificates from 1855 had this information (a lot more on the starting year) so either Ann, James, Douglas or Gordon's should help you out with a date.

Hope this is useful,
hume24