From the police's perspective, they've done absolutely nothing wrong. The father had but they've simply been attacked by a madman.
So why would they be arrested?
Well, they were a part of the whole scheme too. The son's voice over at the end states that he heard the words forgery, trespassing, foul play, self defense in the courtroom, but were lucky to get away with probation.
shareBecause the mother actually killed somebody. Yes, it was ruled justifiable self-defense in the end, but even then I don't think any country would not put you through court after you've caused the death of somebody, however justifiable it was.
shareIn clear cases of self defense the police/DA often don't bring charges at all.
Here it was probably because the family was all employed but their employers didn't know they were related.
The mother killed the madman. There would be a court case and as the movie said they got off on self-defence.
They also forged a bunch of documents to get hired, which is a crime.