The answer is pretty straight forward since CIC has an established process for dealing with caps. CIC will do the same thing for parent/grandparent sponsorship application as they did with FSW applications (no job offer) in 2011 and 2013.
Applications will be opened on a FIFO basis. Once an application is opened, it will be reviewed to confirm it is complete (i.e. to make sure all of the paperwork required is there). Note that this is different than the file or sponsor being approved. CIC will not review and approve the file as part of this first pass - just verify the application is complete. If the application is complete, it will be counted as part of the 5K cap and pass on for further processing. If the application is deemed incomplete, it will be returned without further processing. Once the 5K cap is reached, all applications will be returned unprocessed. And yes - not all of the 5K applications included in the cap will eventually be approved. A certain percentage will be refused for various reasons that fall outside of the scope of the initial review. And yes - this doesn mean that the eventual number of applications approved will be less than 5K.
Because it will take time for the completeness check to be performed, it's likely that any cap counts on the CIC web site will be 1-2 months behind.