It is difficult to say whether you have grounds to challenge anything, but it is certainly a situation where I would encourage you to retain an attorney to investigate further. If your grandfather had a will or trust, those documents will control the disposition of the property, but without more information I can only speculate on the existence of the documents. My speculation would lead me to believe he had a trust in place which transferred the assets to his partner at death, and if there was no probate estate there would be no reason to record the will. Since you state he had a house in Florida I assume that is where he lived at the time of death. If so, you should locate an experienced estate administration attorney in the county where he resided and have them 1) determine whether there was a will, 2) get a copy of the deed for the house, and 3) determine whether you have any basis for further investigating a challenge to the distribution of the assets.
If your (maternal) grandfather lived and died in New York, and there was no estate proceeding brought in any jurisdiction concerning his estate, you would have the right to file in the Surrogate Court for Letters of Administration. Once appointed, assuming there was no valid marriage and the property was not devised to the companion, you could undertake any proceedings necessary to recover your grandfather's property. I represent individuals in contested estate proceedings and related litigation.
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