If there was an agreement between mum and dad as the daughter in-law admits, then it can't change unless both sides agree. 

You're walking on along the street with a friend and you both see a $1,000 bill on the sidewalk. Since the two of you saw it at the same time you agree to split the money. Your friend says he will take it to the bank to get $500 for each of you. Great say you. When your friend gets home he realizes his fridge isn't working and decides to spend the $1,000 on a new one. Does he owe you any money?