how is penelope and telemachus strong because of their relationship?

I concede that both of these characters display strength, but I don't think either has gained that strength because of their relationship with one another.

The relationship between Penelope and her son is strained. We have a woman who believes she's a widow and a group of suitors who see her in the same way. Then we have her son, who is worried that one of these suitors will take a part of his inheritance...... and even worse he sees these same suitors eating up (literally) the wealth of the household.

When Telemachus finds out his father may still live and leaves to save him, their relationship becomes even more strained. He relieves his mother of the place she's taken as head of household, and he reminds her of her true duties, “Go then, within the house and busy yourself with your daily duties, your loom, your distaff, and the ordering of your servants; for speech is a man’s matter, and mine above all others—for it is I who am master here” (8).

And later, after he is reunited with Odysseus, he joins with his father in tricking his mother (Odysseus is disguised as a beggar), in order kill off all of her suitors. Up until now Penelope has had control, but now that her son has reached manhood we hear, “Go then, within the house and busy yourself with your daily duties, your loom, your distaff, and the ordering of your servants; for speech is a man’s matter, and mine above all others—for it is I who am master here” (8).

Even after Penelope sees her husband has returned, Telemachus continues to berate her and take his father's side, and the question here will always be "why?" His mother has never married, she has turned off suitor after suitor, she's made sure that the family honor has stayed intact, and for twenty years she has mourned Odysseus' absence, but that is not enough for the man her son has become.

Her shyness and the fact she doesn't jump into his father's arms is seen almost as a slight, “Mother—but you are so hard that I cannot call you such a name—why do you keep away from my father in this way? . . . No other woman could bear to keep away from her husband . . . but your heart always was as hard as a stone” (246). Again, we see strain and blame.

Si in direct answer to your question, I am goung to say that the "strength" of these characters is not due to the mother/son relationship...... but rather due to their lack of a relationship.