What was the main purpose of your ancestor's letter? What were its secondary purposes?
Is your family missive primarily a conveyer of news, or a letter of condolence, love, rejection, recommendation, thanks, congratulation, consolation or commiseration. Each of these kinds of letters had its own special characteristics and in the Victorian period, there were published books advising people how to set about writing letters of each kind. Letters also rarely served one purpose only, so look out for the moments where the purpose of the letter changes.
It is always very helpful when examining a letter from the past to read it out loud - especially if the purpose isn't immediately obvious.
Tracing Your Ancestors Through Letters and Personal Writings