Video footage online showed the Spaniard, in a yellow jacket, chatting and helping to close the driver's door on his father's Audi after Carlos Sainz Sr (60) had stopped to repair the car's left rear brake.
Stewards decided to take no further action after investigating the helping hand, which happened on Tuesday, following a report from the FIA's sporting delegate.
"The appearance of a son, being near a competitor's car and helping to close the door is not an initiative of the team or driver," the stewards said in their reasoning published on Thursday.
"Since this was not done on the initiative of a competitor or a driver, it cannot be recognised as a violation of the regulations."
The stewards noted, however, that "the behaviour of all members of the motorsport community should not raise any doubts".
"Of particular importance is the fact that very well-known people in motorsport set an example for everyone else," they added.
Three-times Dakar winner Sainz Sr is fourth in the overall standings led by Toyota's reigning champion Nasser Al-Attiyah (52) of Qatar.
