What proof does your husband have that he bought the property and the business 40 years ago?
You say “for business and immigration reasons”. Please explain.
If your husband has proof that he paid all the money in respect of the purchase of the property the business and that no money came from anywhere else, and that proof is sufficient to satisfy a court (if it got that far) then there is no claim to be made by the other two siblings.
My suggestion would be to send them a copy of the initial purchase documentation telling them that whilst the property may have belonged on paper to his mother and father, they did not provide the purchase monies and hence held the property on trust for your husband.
The claimants may be able to argue that the father and mother held the initial purchase price on trust plus any increase in value attributable to property rises but if mother and father promoted the business, that they would be entitled to some of that goodwill.
However, if husband is the administrator of the estate and he is not inclined to let them have anything, they’re going to have to take him to court and the court will have to decide the issue.
Can I clarify anything else for you?
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