Can siblings agree to split their mother's estate regardless of whether one or both are the named beneficiaries in her will?

My mother is secretive about her will. She frequently insinuates that she changed her will. She drops hints that she is leaving the house to only one of us. My sister and I are tired of all the drama. We want to agree to split her estate. Can we make a legal agreement now, while she is still alive, so that when she dies we will split the estate regardless if only one of us is a named beneficiary? If we do not make an agreement before she dies, can we make one after she dies? After she dies, if she leaves her estate to only one of us, can we nonetheless agree to split the estate ? Would there be tax implications? The will would be probated in New York.