What caused Danni's resentment of Finn in the novel, Tell the Wolves I’m Home?

When Danielle and Finn were children, they were very close. When Finn left home, however, Danielle felt abandoned. She resented the fact that he left and managed to do all of the things they dreamed of. He became a famous artist, while she became and married a tax accountant. Part of her never forgave him, but she took all of her out on Toby when Finn finally returned with him. Danielle blamed Toby for Finn's illness, for taking advantage of her brother, and she held him responsible for corrupting Finn's gentle nature.