Their argument is that when your Dad died, you became the beneficial owner of the property and are enjoying the benefits of the property even though title has not passed to you. There is some caselaw that might support this claim. It can be challenged. However, you do have to see a lawyer to get more advice about this. Do not ignore any suit naming you as a defendant.
The foregoing answer is for informational and educational purposes, not for purposes of legal representation. This answer is based on New Jersey law and is necessarily general in nature.. Laws in other states may be different, and each situation is different, so this answer might not apply accurately to you. No attorney client relationship is to be implied from this answer. Always seek independent legal advice.