The rule is very simple indeed. If 'uncle' is part of a name, used like a title, or used instead of a name, it needs to have a capital letter. Otherwise, it has a lower case 'u'. I always write a letter to my uncle on Sundays. I always write a letter to Uncle Brown on Sundays. 'Good night, Uncle.' Mary said goodnight to her uncle before she went to bed. The same goes for all family members: I went shopping with Mum and Dad. I went shopping with my mum and dad.