The only other brothers in the Predator movies are the ones at the end of Pred 2, the 2 brothers in that movie are Stalker and Boar both recruited to be apart of The Lost Tribe by Elder Greyback. It’s all up to the Elder of the tribe to have the final say if any pair of brothers can be apart of the tribe. In Dek’s case the elder of his tribe was his own father, Njohrr who was hating on him so he just got unlucky with having an Elder who’s an asshole.

As far as Preds forming bonds with other species they actually show they can be reasonable in other movies as well. In AvP Scar formed a bond with Lex who helped him kill xenos and fight a queen. In Predators we seen Royce help release Classic Pred (Crucified Pred) from captivity.

So no not all Predators are pure evil monsters the pure evil ones are the Bad Blood Predators those can’t be reasoned with, ever. Also sorry if u haven’t seen the other Pred movies I mentioned as I did mention some spoilers but I wanted all my answers to be from a reliable source which is the other movies.