Hi lowndes and welcome to Cruise Chat.
With security being what it is, as determined by the Dept. of Homeland Security, I can't think of any U.S. based cruise line that allows visitors onboard prior to sailing. Everyone who boards must have submitted information prior to boarding, like passport numbers and other identifying information. This information is checked to make sure the name on the cruise ticket matches exactly the name on the passport or birth certificate.
Even "official" visitors must go through that screening process. I would assume that the cruise line will not allow relatives to accompany you onboard unless they're sailing with you.