The fact your sister is with this guy tells me she is one of those white trash types the gets married at a young age and starts getting into arguments soon after.
When they were together, he messaged me on facebook several times asking me if she was with some other boys.
This kid obviously has some insecurities.
Also, you're probably right about the married young living in a trailer park kind of thing.
Funny story: my other little sister (10 yo.) got on my father's facebook and sent the kid several messages calling him an starfish and not to talk to my sister again.