What is the big hint that Westing gives in the tenth instruction?

TENTH ♦ Each pair in attendance will now receive an envelope containing a set of clues. No two sets of clues are alike. It is not what you have, it’s what you don’t have that counts.

In context, this alerts the heirs to the fact that it will take more than the hint they are given to solve the mystery. They will need to observe, learn, and use that knowledge.