When are gift/estate taxes paid on a property the was given using a life estate deed?

My Dad just passed away in December and about a year ago when he got real ill he had his attorney write a codicil to his will to give the shore property (a condo hotel room) to my sister. He did not realize and apparently the attorney didn't ask or check that the deed is in both my mother's and father's name with right to survivor. So as I understand it the property went right to my Mom and never became part of my Dad's estate to give it to my sister. We don't want this property to have to go through probate when my Mom passes, so we are thinking a life estate deed from my Mom to my sister. We don't want anyone to have to pay gift or estate tax now, can my Mom's estate pay the estate tax when she passes. Mom is 89, the property is in New Jersey and my Mom and sister live in Philadelphia. My Mom's assets including the property as of today is about 1.5 million