What would you do?

my son and his girlfriend sherry live together they have been together less than a year. my 2 sisters will invite them both to family gatherings and holidays. sherry has been caught several times lying on members of my family she also adds to and takes away from the truth causes scenes and drama when she isn't the center of attention. i live out of town and travel to my sisters homes for gatherings and holidays. i'm tired of sherrys antics she pulls to be center of attention. how do i tell my sisters how i feel in a firm but friendly way something has to give here