There is something pretty weird about that. Maybe it´s her axiety (?), maybe when she vists her parents it goes up because she is reminded of unhappy or painful moments. Maybe her family is or was kind of toxic, it happens, a toxic family does not need to be openly abusive, (like having an alcoholic parent or something like that), sometimes parents can torment children without knowing for years and years, or knowing but not caring while looking like a completley normal and happy family. Also, she is very very close to Joe -which is a good thing because he doesn´t treat her as a princess- and that kind of closenes between sibilings -sometimes- can be the result of sibilings growing up under some time of abuse or pressure .
Sometimes, those kind of things can brake a family, even when time passes and everything is spoken it is impossible to forgive completley, so maybe visiting her parents once or twice a year is what is better for her and for them in order to keep the family "conflict free".