I will be officiating my cousin's wedding...It is not formal...I have been told to wear just a white dress shirt with my tartan tie. Others have said no.
What's your cousin say? It's their wedding, after all. If the bride says informal, then informal it is; no coat, and perhaps no tie. Never hurts to ask for clarification, but it's her big day. Respect her wishes.

A complimentary blue Oxford shirt would look good with Blue Ramsay, or perhaps a blue shirt with clerical collar.