what is the "promise", and is this promise kept?

Ruth promised Dee-Dee she would stay home, that she wouldn't leave her. No, that promise was not kept.

"According to Ruth, Dee-Dee was very smart and confident, and got good grades. No one ever made fun of her: "she was the first American in my family, while Sam and I were immigrants." Tateh was proud of Dee-Dee, too, which made Ruth a little jealous. The two sisters were not that close, so she knew that Dee-Dee's request carried great weight. Although Ruth promised that she would stay, in the end she broke her promise: something that Dee-Dee never forgave her for."