To avoid the appearance of any conflict of interest, the seller and seller's agent should have disclosed that they are siblings.As for the hidden defects, you can sue the seller for fraud.  Fraud is the intentional misrepresentation or nondisclosure of a material fact made with knowledge of its falsity and with the intent to induce your reliance upon which you justifiably relied to your detriment.In other words, you would not have bought the house had you known of the hidden defects.Your damages (monetary compensation in a lawsuit) for fraud would be either the benefit of the bargain or your out of pocket loss.Benefit of the bargain means a defrauded purchaser may recover the difference between the real and represented value of the property purchased regardless of the fact that the actual loss suffered might have been less.Out of pocket determination of damages for fraudulent misrepresentation permits recovery of the difference between the price paid and actual value of the property acquired.