The heroine was a girl in her early to mid teens. Her father was some kind of wealthy contractor. Her mother was an ex model or beauty queen. She had a younger brother who had “issues” that possibly involved mistreating animals.
Anyway, her grandmother (mother’s mother), no longer lived in the US, lived in Canada where there was no such thing as an “Uncanadian Activities Committee” and the grandmother was estranged from the girl’s parents. The girl’s mom had started winning beauty contests as a young girl which was why she didn’t turn out progressive like the girl and her grandmother.
The grandmother and the girl had a ruse going where the girl went every summer to what her parents thought was a special camp in Canada, but it really was just going to spend the summer with her grandmother. The younger brother had been on “the waiting list” for the camp for years, but of course, never got in (the implication was that his issues were more like being evil than having psychological problems).
Anyway, parents were paranoid anti-Communists, and the father had built their house (in Michigan maybe? Wisconsin) with a sort of safe-room area where the family could escape in case of nuclear war/communist invasion. The girl comes back from summer “camp” and finds there is a handsome boy living in the safe room— maybe a Vietnam era draft dodger? She has to bring him food etc, and make sure no one figures out he’s there.
I think this was probably published in late 1960s or early 1970s. Suggestion: The Weedkiller’s Daughter by Harriette Simpson Arnow