Spoilers The grandparents leave without saying goodbye to Margaret after dinner. Its not major just implied. The mother was abandoned by her parents and they no longer talk to her.
An adult whom had been disavowed by her parents reaches out to them after a decade. She doesn't explicitly forgive them but she's willing to give them a chance.
It's suggested that Barbara's (the mother's) parents are to some extent abusive as they abandon her after she marries a Jewish man as they claim they can't bare it because they are extreme Christians.
This is one of the main themes of the movie. The main character is trying to understand religion and work out where she fits in the world, whether she wants to follow a religion and if so which one. Her father and grandma are jewish and her mother's parents are very Christian.
The movie is about a girl working out who she is and in the process she spends a lot of time trying to work out what religion she believes in. So I guess you could count that as existentialism?