What is the ruling of Islam regarding an unmarried woman writing a deed for property she owns to her divorced sister's two sons due to her having no one to support her?

If the mentioned writing is not executed until after the death of its owner, then it is a will, and its ruling is that of wills. Thus, it would be within the limits of one-third for the sister's children, and what remains after the one-third would go to the heirs.

However, if it was a complete gift with its conditions fulfilled, and she relinquished control over it, and her sister's children or their guardian took possession of it as an owner would, and she was of sound mind at the time of the transaction, and there was no intention to harm the heirs, then there is no legal impediment to it; because a person has the freedom to dispose of their wealth in a lawful manner.

But it appears that this writing is neither valid nor enforceable, because it is neither a complete gift nor a specific will. The prohibition would be even stronger if the intention was to harm the heirs. If the woman dies, this writing would be considered equivalent to a will.