I agree with you that the tone of Chopin's "The Storm" is sympathetic.
One place to find evidence to support your conclusion about tone is in the description. A speaker's description of characters will usually reveal his/her attitude toward those characters.
In paragraph twelve, the speaker describes Calixta as follows:
She was a little fuller of figure than five years before when she married; but she had lost nothing of her vivacity. Her blue eyes still retained their melting quality; and her yellow hair, dishevelled by the wind and rain, kinked more stubbornly than ever about her ears and temples.
The speaker describes her character as vivacious, and the character's eyes melt. The speaker uses specific detail--"about her ears and temples"--to give her character verisimilitude, or realism, and to create imagery; to make the face concrete. The speaker encourages the reader to imagine Calixta in her vivaciousness.
In paragraph twenty, when Calixta staggers backward, she is "encircled" by Alcee's arm--a protective, tender motion. When she cannot compose herself,
Alcee clasped her shoulders and looked into her face. The contact of her warm, palpitating body when he had unthinkingly drawn her into his arms, had aroused all the one-time infatuation and desire for her flesh.
The moment is described positively. Again, specific detail creates imagery--Alcee "looked into her face"; her body is "warm" and "palpitating"; his infatuation and desire are "aroused." The imagery makes the movement concrete, as well as adding verisimilitude. And the images are positive and tender.
This is the moment that leads directly into what society calls adultery, but the speaker considers fulfillment and necessary passion.
The speaker's description reveals the tone to be sympathetic.