Annulment of Contract

our property (a house and lot) was entered into a pawn contract by my mom last year, she was 72 years old then. we feel that the contract was intended maliciously because the pawner included my sister's debt to them (who passed away a year before the contract was executed). the actual pawn cost was 1 million pesos and they added the 500 hundred thousand pesos debt of my deceased sister to make it appear that my mom received the total amount stipulated into the said contract. every month my mom is paying the interest which is exactly the same amount as what she is receiving with her pension. now she cannot buy her personal needs as well as her medication. this is our dilemma, considering that my mom is incapable of paying the dues they have set because of her old age. we are willing to pay but only what is due amount my mom actually received less the interest incurred because we felt that the contract is unjust as much as it is unlawful.

my questions are;
1. is this a ground if we will file for an annulment of contract?
2. does the law allow a 72 year old person to enter a binding contract?
3. what is the possible legal remedy for this problem?