It looked like they hinting that the other brother (who later overdosed) did it and the family knew about it. Pretending that the French guy was their son just drew attention to them though, he looked nothing like him and was 7 years older with a French accent.
Something fishy in the way the brother said "Good luck" when first meeting him, although that could be a lie. Also the sister who got told he was an imposter over the phone then turned up and acted like nothing had happened, all points to one thing.