This question is rather confusing. I am not clear if these instructions were part of a will or a trust or just general instructions? To the extent that these instructions are not a part of a deed, will, trust or other legally binding document they may have no effect. I think you would be well advised to seek the advice of an attorney to deal with the estate(s) of your parents.
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No attorney can answer this without seeing the document with the instructions. If it is in a Will, that no longer matters if the Will was not filed for probate within one year of his date of death. It if is a deed, an attorney must see that deed, but that could be enforceable. If it is merely a letter of instructions, it is not binding and:
1. the current owners can sell the land
2. the current owners can sell any part and use the proceeds as they wish - pay taxes or otherwise.
3. the current owners can agree to do something else.
This all assumes the "instructions" are not in a recorded deed.
Notice I used the words "current owners" which may different from what you termed "compulsory heirs."
This comment does not create an attorney-client relationship. The law and its application by the courts is constantly evolving and changing. This discussion is not to be taken as a definitive guide, and should not be relied upon to determine all fact situations. Each set of facts must be examined separately with the current case and statutory law analyzed and applied accordingly.
I agree with the other two attorneys in that without the documentation in front of me, the inquiries are difficult. I can only imagine your parents had a trust with provisions determining the disposition of the property. The trustee(s) must adhere to the terms of the trust until and unless all of the heirs petition him/her to act as they wish. Even then, if the trustee wants to hold fast to the terms of the trust, you may have a fight on your hands.
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