The fact that he says "BEFORE it gets here" suggests that he expects that the fire would get there, at some point. It could be just that it is moving closer, so having it reach the house eventually is an interpolation, but I prefer the explanation that Emil has seen the fire reach the house at some point, in reality.
Anyway, the mansion burning down is a good way for his family to lose their money. Although I am not sure how this would have affect Torb and Siv, who obviously did not live there. I suppose they might have been receiving a stipend from Emil's dad? Also, the timing really does get confusing, if the fire was set by Helga: if she was alive while Emil ate his lonely meals, why wasn't she there or even mentioned? (I suppose it could be that she was also a pretty neglectful parent, to the point that Emil and Emil's nanny stopped expecting her to show up.)