Is the sister entitled to claim the deferred mahr and maintenance after her divorce, or does concealing her mental illness invalidate the marriage contract from the outset?

You should not have disclosed the illness of the sister mentioned, because this illness does not necessitate an option for her husband (to annul the marriage) if he had not stipulated safety from it, nor does it affect conjugal relations unless it reaches the stage of insanity. Not mentioning her psychological illness does not forfeit her right to maintenance and housing during the waiting period (iddah (definition)) — if her divorce was revocable — in addition to her right to the deferred dowry and compensation.