And yeah, if she's not the host, telling her son that he has to go to Auncle Host's house and deal with Cousin Silent Treatment is not going to fly. Niece is old enough to have a 16yo daughter from a marriage; even if she started at 18, that means that the middle generation in this story would at least be in their thirties. It's a time when a lot of people see less of their cousins, unless they specifically get along well with them. Having a holiday dinner with LW's branch of the family and the people who are speaking to them--whether it's family or friends--would be totally normal.