No legal requirement for non professional sellers but wilful misrepresentation and omission is illegal in NJ under the consumer fraud act which covers real estate and the broker is liable. Bummer that this happened to your mom and not sure when it happened, but if it happened in NJ after 76 and she bought from a licensed broker she should have been protected. If you're buying from owners, all bets are off. So yes you can be dodgy but the broker can't

Also as someone who bought a home recently, nowhere near a flood zone or a source of water, a flood assessment was still required by the bank (even though the bank didn't require the appraisal and viewed it as a low risk mortgage). So today of you're buying through professionals and using a reputable mortgage lender, you should (extra emphasis on should) be covered