Need your sound advice?

my son has a girlfriend, sherry is her name. she seems to think at all family gatherings and holidays everyone should make her center of attention. rencently at a family gathering held at my sisters home she caused a scene verbally attacking my niece accusing her of ignoring her. when my niece was trying to get her 3 children ages 8, 4, and 8 months ready to go home april first. i'm to the point i'm not sure what to tell my family when i know sherry will be there my nerves are shot at the scenes she creates as she isn't the center of attention. thank you for your adivice.