Grandfather ped away. Beneficiary to his estate ped away a year ago. How will this work?

My Grandfather ped away last week and left behind a sizable estate. My mother and uncle were beneficiaries to his estate before my mother ped away in June 2006. I have recently found out that at some point after my mothers death the will was changed so that only my uncle was to inherit his entire estate. My grandfather was diagnosed with Alzheimers in 2007 and moved to a nursing home later that year. so I am uming it was done prior to the diagnosis. My uncle was given power of attorney over my grandfather and had control of his affairs up until he ped away in June 2009. At that time my cousin began handling his affairs until my grandfather's passing last week. He will also be the executor of his estate. It has always been known by everyone in the family that my grandfather wanted his estate to be divided equally between my mother and my uncle. I am assuming the will was changed in good faith and my uncle, had he survived, would have distributed the money fairly to include my sister and I. My question is, since it now appears that my cousin is the lone heir of my grandfather's estate. Do we have any grounds to challenge the will? I suppose it is possible that my grandfather was not of sound mind at the time of the will being changed. Also, My cousin who will be the executor of his estate is a convicted felon for embezzlement in 1986. Does this have any bearing on any of this? I am aware that life is not fair, and no one is entitled to anything, but it seems that my sister and I are being left out just based on my mother passing away before my uncle. I am just wondering if we have any options.