Thanks for all the work and prompting.
Positive result from Georges marriage certificate. Parents William Cowie and Margaret Martin, so definitely the brother of my Alexander. The marriage is in Fetterangus so is it safe to presume that this is the George who is with Isabella Brown with his brother William in 1851? Where then is Alexander (Ugie Street with the duplicate William?) Can I presume then that the birth of William Cowie to Andrew Cowie and Isabella Brown is their father. The OPR confirms Andrew (Mains of Pitfour) and Isabella (Toux) are both unmarried, witnesses Samuel and William Brown of Toux. (I have seen this certificate before but it is not on my saved searches for some reason)
The sibling William is very elusive cant find anything about him until there is a death at sea in 1890. My Alexander was also a merchant seaman so there is a good chance this is him.
If William is illegitimate I dont think the family on Kirk Street are Williams brother I wondered that as well but there are so many Williams, Georges and Alexanders it might be a coincidence.
I will see if I can find out anything about Andrew.