French Relative Pronoun: Qui

C'est celui qui le dit qui l'est: childish sentence that means: the one who says the insult is the one who deserves it.

Qui is invariable.
It is a relative pronoun: it is used to add something (usually extra information) to the noun it follows.
C'est la pluie qui fait pousser l'herbe.
C'est la fille qui arrose le jardin le dimanche.
The piece of information may not be the main one:
La fille, qui arrose le jardin le dimanche, travaille au supermarché.

When the relative pronoun is used with a preposition, apply this simple rule that prevents you from making any mistake:
if the antecedent signifies people, use qui.
C'est la fille avec qui je vais au cinéma.
In any other cases, use lequel or one of its derivative.
J'aime la maison dans laquelle j'habite.