Is it permissible to annul the sale of a house that was sold for 1,400,0 Riyals, and it later became clear that its true value is 2,0,0 Riyals, considering that the sellers have not yet received the payment, and the contract was signed without their knowledge of the property's true value? And can an increase in the price be requested to complete the sale?

The sale is valid, and there is no deception, as long as you requested a specific price before finalizing the sale. Therefore, you have no right to the option of ghabn (gross deception). As for your mother, if she did not appoint an agent, then your grandmother's sale of your mother's share is contingent upon your mother's approval. If your mother does not ratify the contract, then the sale is void and annulled concerning her share. The sale is valid regarding your grandmother's and your aunt's shares, but void regarding your mother's share, and the buyer does not have the option because he was aware of the situation.