It sounds to me like you may need to get court intervention on this one. How is the property currently titled? If it is just in dad's name alone, then you need to probate dad's will or, if he had no will, administer his estate. This will resolve ownership which you will need to do to sell the property. Many states will give you a document (a release of lien in NY) which tells purchasers of the property that all estate taxes (if any) have been paid. The title company of the future purchasers will want to see this.
If dad's estate has already been administered, then title to the property should have been resolved. Contact the attorney that handle the administration or the executor/administrator of the estate for answers to your questions.
If the property is owned by you and your two siblings, you will need to work out something with them concerning the sale of this property. Even if you have good intentions, you will be unable to sell this property if you only own 1/3 of it.