what type of lawyer do I need

My Father's will was drawn up and discussed with his children years before his death in Nov.2018. All assets at time of death to be shared evenly. Unfortunately, four years before his passing, he became mentally and physically weak due to dementia . I have just discovered my sister, who resided with him, had him sign over his $600,000 house to her less then a year before his death. Where do I go from here?