Bruno was suspicious of Felix and Agustin at first, let's be real. He's looking out for his sisters, and himself because people in the town didn't generally try to befriend him (at this time they were already distancing themselves from him and his prophecies).
But they tried being his friend before they fell for Pepa and Julieta.
Felix was local, his family little known, and once he randomly asked him to look into his future. Bruno, slightly confused because had Felix not heard that he killed someone's goldfish?, but he does it. It's a rainy day in the vision, wind blowing and clothes fluttering, but Felix and Pepa are clearly in it, clearly happy despite the weather bearing down on them. Bruno finds relief that it's the first happy prophecy in a long time, and Felix, sheepishly, asks for his blessing in courting Pepa. Bruno says it's up to Mama, but smiles in a way that lets him know he knows he has Pepa's best interest at heart.
Agustin is a little more difficult to read because he's an outsider. He tumbled into town one day, falling down the mountains, in a way befitting a clumsy man. Bruno is there when they bring him to Julieta, covered in bee stings and thorn scratches and he notices the delusional look on the foreigner's face as his sister feeds him an arepa. It's not the usual 'wow thanks' look that townspeople give her, but a look of wonderment as though he discovered the holy grail.
Agustin is allowed to lodge with a man down the road, but comes back. Bruno sees the way each time it's for even just minor things like scratches, but his immediate thought is to find Julieta. Julieta is the town healer of course, but his intentions change each time. Mama looks at him with suspicion - he bears the same kind of skin as the murderers who killed Papa, is from the outside world. Of course she's suspicious, but Bruno, quiet Bruno who can slip between the cracks in the house walls, can tell that Agustin only has Julieta's best interests at heart.
One night after dinner, Agustin helps him with the dishes.
"You love her, don't you?"
Agustin blanches in a way that throws his glasses askew, bringing a soapy hand up to adjust them as Bruno continues idly wiping down dishes, looking at the plate in his hand nonchalantly.
"You love Julieta, don't you?"
"W-well I suppose I..." Agustin fumbles for his words, much like he fumbles for steps. "How did you figure it out?"
Bruno gives him a deadpanned look. "Not a lot of people hang around me just for me. I don't know if you've noticed but nobody talks to Bruno just for Bruno."
"...I'm sorry." Agustin sounds genuinely sorry. "Your mother doesn't like me very much and your sister Pepa and her husband..."
"This isn't about them." He placed the plate down, turning to face Agustin full on. "Do you love Julieta?"
"Then that's all that matters." Bruno turns his attention back to the dishes, calm and quietly high fiving himself in his head. His rats do it on his shoulder. "But if you hurt her, my rats will poop in your food."
That was how Bruno became a tio.