Maybe your sister was thinking of this from the SSA web site:

It may be to your advantage to have your Social Security benefits start in January, even if you do not plan to retire until later in the year. Depending on your earnings and your benefit amount, it may be possible for you to start collecting benefits even though you continue to work. Under current rules, many people can receive the most benefits possible with an application that is effective in January.

http://www.ssa.gov/pubs/10035.html