] I always wondered why it took Col. Brandon so long to tell any of the Dashwoods the story of his ward.

I think he explains why he took so long to tell his story when he says:

'...When I came to you last week and found you alone, I came determined to know the truth; though irresolute what to do when it was known. My behaviour must have seemed strange to you then; but now you will comprehend it. To suffer you all to be so deceived; to see your sister - but what could do? I had no hope of interfering with success; and somethmes I thought your sister's influence might yet reclaim him....'