ANSWER:
Such an agreement is void and has no value once they die. They are sinful in distributing it like this as if it is a gift in their life time, they must be fair and equal between their children and give them equal shares.
Whatever is agreed upon before death becomes void once they die and sharia law must be implemented.
Your husband is sinful for preventing you from visiting them and he is also acting weird as it is none of his business what goes in your family unless he has an eye on your share!
I advise you to keep all of your affairs with your family private and not share it with him and not share anything of your financials with him.