Whose house is Love on the Corner? Why does Xin Lei have a key?

Xin Lei's mother and Qin Lang's mother were good friends when they were young. They bought them together. But then Xin Lei's mother married in Shanghai and still kept the key to the house. Later, Qin Lang's mother died. No one knew that Xin Lei was the daughter of someone who bought a house with Qin Lang's mother, so there was a later story. ...