What are the roles of social taboos in both stories "The Fall of the House of Usher" and "The Minister's Black Veil"?

A taboo is defined as a forbidden or prohibited practice. Sometimes, social conventions also prohibit connections with certain places, people, or things. These elements are considered taboo, like those in the two stories you mention.
In The Fall of the House of Usher, the social taboo in question is incest, and in The Minister's Black Veil, the social taboo is the collective sin of a congregation. In both stories, the taboos are only hinted at. In The Fall of the House of Usher, Poe highlights Roderick's obsession with the house and his fate. The Usher genetic line faces extinction because the incestuous connection that preserves the union of family members and protects the vulnerable Self against the encroachment of Others is disintegrating ("...very remarkable fact, that the stem of the Usher race, all time-honoured as it was, had put forth, at no period, any enduring branch..."). Poe uses the social taboo of incest to explore how sexual perversion protects the dysfunctional reciprocity between the members of a social group while shielding the individual against engagement with the larger society.
In Poe's story, Roderick is obsessed with the idea of the house as a sickly structure. Symbolically, the "extensive decay" surrounding the house is indicative of the spiritual decay of the inhabitants within. Roderick knows that a crisis is at hand; that is why he summons his friend, our narrator. However, he's not quite ready to relinquish his tenuous hold upon his relevance. Madeline's supposed appearance after she's been entombed is noteworthy. While Roderick attempts to reconcile his reliance upon a closed system with his desire to affirm his humanity (by social interaction with outsiders), Madeline rejects the idea of relinquishing her morbid connection to her brother altogether. She attacks Roderick, and both die in the terrible struggle.
Thus, the social taboo of incest in Poe's story highlights the struggle between the Self and Others. Roderick knows that the unnatural link that connects him to Madeline will destroy them both, but he is reluctant to renounce his only claim to personal relevance and significance. In the end, the incestuous alliance destroys the siblings, and the house symbolically falls in upon itself. 
In The Minister's Black Veil, Mr. Hooper's veil inspires suspicion and fear in the hearts of his congregation. Even his fiance, Elizabeth, begs him to remove the veil in order to staunch the sickening rumors that threaten to engulf his ministry in scandal. For his part, Hooper declines to humor her. Instead, he preaches about vague secret sins, the kind people hide from themselves, their families, and their friends. Someone in Hooper's congregation surmises that their beloved minister may have engaged in a forbidden relationship with a young woman. In the absence of fact, prejudice and misconceptions hold sway among Hooper's bewildered parishioners.
Hooper's veil becomes a social taboo. Ironically, it highlights other social taboos: the secret sins no one wants to admit to, whether they be sexual sins or otherwise. Hawthorne uses social taboos in his story to highlight mankind's collective sin. No one is immune from the ravages of spiritual decay, not even a minister. All are equally guilty ("I look around me, and, lo! on every visage a Black Veil!").The veil accentuates the repulsiveness of sin. Because of his veil, Hooper experiences isolation and emotional exile from his community. The veil and the social taboos it represents (the collective, private sins of a community) invite readers to contemplate the spiritual divide between men and God. 
https://www.eapoe.org/pstudies/ps1970/p1977201.htm