I don't even understand what would have made you decide to basically stalk him by looking up his record. I am sure that your sister is fully aware of that if there's been anything new within the last few years - she's been living with him and in a relationship with him, she'd know if he was arrested or charged.

You're still making loads of sweeping generalisations and assumptions about him. Until you ask him (which you have no right to do) you will not know why he was at that school. How do you know that his family didn't choose to put him there because they wanted him to have a good education? You don't. You cannot assume that he was expelled from a bunch of other schools. It's your sister's job to deteremine if she is ok with his wishy-washy lifestyle or not. Clearly she is ok with it because she's moved to a new state with him...