First of all, once Omegas find their mates, they have their own pack/family unit and that is respected. Secondly, humans are cared for and revered by werewolves (you'll see this more illustrated in further chapters). In the case of an Omega and human couple, the Omega is the one that protects the human under the assumption that, some day, the human will choose to be turned by them and take on the more dominant role (be it male, female, or intersex). In the case of Maribelle, her moving in with him is perceived as if she's going to a human area to watch over those humans and report any vampires. Both werewolves and vampires adore humans, it's what the turf wars were all about years prior. In a way, humans are like a bounty to them, ha ha. Treasure.
He's not moving them in. Mari's doing all the leg work. She just told him "I'm telling my parents and I'm moving in." The reason that Maribelle did not bring him right away is because the guy has little to no idea about anything dealing with werewolves. She hasn't prepared him yet to face her strong, beloved cousin and his equally as strong werewolf mates (not that they'd do anything to him). Once again, humans are revered and protected, so she wants to ease him into it. "You see, my cousin and his mates are all gay and they're all total sweethearts...I promise they won't eat you" I'm sure it took her quite a bit to explain what being the mate of an Omega entailed, to start with. We will see him later on, though, generally acclimated.