
GM Forever |

This is definitely a bit of a hard one, huh? I will try my best to help though.
So, if they are born into the house, they could be viewed as little more than a bastard, someone who they acknowledge but only as a bit of a "well, yeah, but he doesn't represent the House you see" sort of fashion. If they were adopted, then it would lead to a scenario of, as you said, why? Perhaps his family died protecting the House from some beast or served them beyond the mere bounds of loyalty and so they adopted him to honor that?
Being an inquisitor is also very much a calling, beyond that of a mere cleric or warpriest but maybe not the same level as paladin. However since Erastil is his patron deity, perhaps he fled to the church to get the sense of belonging his family (real or adopted) never could show him. Not so much he was sent away but that he fled to them. Maybe he did it in a fit of teenage angst or perhaps he had run-ins with the church over the years?
Another angle to look at is how does his family view his joining the church? Do they shrug it off, glad that the near-bastard is gone or do they feel slighted from his departure, unaware of how they had been consciously or unconsciously treated him over the years?