If your mother's final wish was verbalized that it carries no weight. You can initiate a lawsuit and force the sale of the property however.
I agree with Mr. Gallo and a division by sale can be court ordered. In the alternative you could ask for an informal mediation to get the situation cleared up with an independent party. That way if you end up in court at least you will have tried to work it out.
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Dear Staten Island Part Home Owner:
In New York the lawsuit is known as partition. You will need an attorney. When you have an attorney on your side, ready to force a sale of the house, it is likely that the brother and nephew living in the house will instead realize that they must buy out your interest.
If the house is owned free and clear, there are still other expense events, incurred in home ownership and if you have not shared in those expenses, the value of your contribution may be charged against your share. Likewise, your brother and nephew are not entitled to live rent free to the economic disadvantage to you and your sister, and they will owe some money to the group for the fair value of the cost of the use and occupancy.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.
You need to provide more information. Was there a Will? How do your names appear in title? Is there any income generated by the house, and if so, do you share in that? Do you share the expenses?
(1) I may be guessing. Do not act or rely upon this info; (2) We have not established an attorney-client relationship; and (3) If you insist I tell you something upon which you can actually rely: don't eat yellow snow.