I have been reading Marge Piercy's books since I was in college. She is a complicated author. She started out with science fiction, and then branched into plain fiction with a leftist political bent, and unusual sexual groupings. Summer People is about a threesome, Willie, a sculptor and carpenter, Dinah, a composer, and Susan, a fabric designer. They live as a family on Cape Code. for ten years, they have lived together in two houses on a pond, seen Susan and Willie's children through adolescence and into adulthood, and been happy.
Also living on the pond part time is Tyrone, a multi-millionaire who flies around the world making deals and more money. Susan is dazzled by Tyrone's possessions, his drive, his apartment in new York, even his daughter Laurie, who confided in Susan when she was a teenager. Willie and Dinah see Tyrone as irrelevant to their lives. He is one of the "summer people," the wealthy types who drive up real estate prices, build huge, ugly houses and eventually go home in the fall, leaving the year round residents of the Cape to get on with their lives.
As Tyrone becomes more attractive in \Susan's eyes, she tries to insinuate herself with him by doing "favors." Dinah and Willie see the favors as unpaid labor. One of the favors almost leads to Susan's death, as she agrees to check on Tyrone's house during a blizzard. A tree falls on her truck. Luckily, Dinah and Willie become worried, and go to find her. They cut away the tree and pry Susan out of the truck.
Dinah, particularly, is enraged by her decision to drive out in a howling snowstorm at the whim of a person who is not really a friend, but a casual acquaintance. Dinah rages at Susan, and Susan throws her out of the family. How the three of them and subsidiary characters such as Tyrones insecure daughter Laurie and Susan and Willie's son, Jimmy, and Itzak Raab, an internationally famous flautist, for whom Dinah has written a piece of music, deal with the breakup of the family is the subject of Summer People.