One of the most memorable songs in Encanto is "We Don't Talk About Bruno," but the rule might not be quite as definitive as the title suggests. Much of Encanto's music was written by Lin Manuel Miranda, like Moana five years prior, and the soundtrack is noteworthy for the fact that the diegetic songs do a lot more work in developing the story than those of most Disney Animated Classics. But "We Don't Talk About Bruno," while it's probably one of Encanto's best songs, might actually be somewhat misleading.
The mystery surrounding Bruno is one of Encanto's key tensions. All that Mirabel knows about her uncle, who mysteriously disappeared when she was only five years old, is that he's a taboo subject among the Madrigal household. Unlike the rest of the family members, whose magical gifts present a clear benefit to the community, Bruno's ability to tell the future made him rather unpopular in the past, according to the song. In this regard, the film's protagonist and her long-lost uncle are rather similar black sheep figures despite one having powers and one not: in Encanto, both Mirabel and Bruno threaten the miracle and are presented as disappointments to Abuela.
It's Bruno's relationship with Abuela that forms the backbone of one Reddit theory which claims that the rule "don't talk about Bruno" only applies when Abuela is around. Thus, it isn't a firm, 24/7 mandate, but more akin to the concept, "don't upset Abuela." Given how Encanto underscores the expectations Abuela places on all her family members, it makes sense that the rule about Bruno would be specifically tailored to keep her happy. Clearly, plenty of family members are in fact willing to talk about Bruno to Mirabel when Abuela isn't around.
This theory aligns well with evidence from the film, as Abuela seems to be the only Madrigal genuinely upset with Bruno. But for all her anger/disappointment, she clearly cares about her son a lot too, which is what forms the tragedy of Bruno being hidden for 10 years in Encanto. Abuela missing Bruno while also being keenly aware of how his gifts alarmed the community forms a solid basis of reasoning for why the family would avoid bringing him up around her. Along with the fact that Abuela is the unquestioned head of the Madrigal household, this helps to explain why the rule "don't talk about Bruno when Abuela is around" would come to be, as a way to avoid upsetting her or getting in trouble.
At the same time, it's reasonable to expect that other adults besides Abuela would want to keep Bruno out of the conversation, which somewhat debunks the theory that Encanto's Bruno rule only applies in her presence. While Pepa, Félix, and their kids open up about Bruno easily in the song, Mirabel's family (except for Isabela) is noticeably absent. Based on Julieta's strong emotional reaction when Bruno reappears at the end of Encanto, it seems the two have a close bond, so she might want to subscribe to the rule as well to avoid feeling the pain of his loss. But more than that, Bruno's disappearance was linked to Mirabel's failure to receive a gift and his vision that she would destroy the miracle, which is information Mirabel's parents would likely want to protect her from. Thus, there's strong evidence that the rule isn't just to protect Abuela's feelings, but Mirabel's as well.
Nonetheless, it is ironic that Encanto's Bruno song not only spoils his twist appearance, but also contradicts itself by being all about him despite the title "We Don't Talk About Bruno." Whether it's to protect only Abuela or other members of the family as well, it's a rule that's been in place for a long time. Thus, Pepa's family's willingness to talk about Bruno appears to indicate that the rule isn't as powerful as it seems.