What legal rights does the executor of a Will have over a person/person's property prior to death?

Hello. I am a Maryland barred attorney and practice in this area.  The NY attorney wisely suggests seeking legal guardianship in a situation like this.  In this state, there are two kinds of adult guardianship - a Guardian of the Person can legally make medical and healthcare decisions and the Guardian of the Property handles the finances (including decisions to sell property).   It is possible to seek both kinds of guardianship in the same proceeding. 

I am sorry for your troubles.  If the legal status is as you say then you all have the same rights legally as things stand now as to your parents.  Your sister and her daughter only have power over the estate when your parents pass away. 

What you need to do is petition the court for guardianship of your parents.  In New York it is done under the mental hygiene law.  As for the right to make the choices both for health related issues and for their financial issues (sometimes also called "estate of the person" while they are alive). It will also help you protect their assets.  It comes with it great responsibility and accountability.  You should go and speak with an attorney in your area right away.  Good luck.