Betty was very close to the Glover family, with whom Biff had a relationship with Linda Glover and when she died they became close, and he moved in with them, as far as i'm aware 'Kepper's Cottage' is Seth's not Biff's.
Biff turned up last night , and discussed with Betty that she should remain in the house for as long as she wants, despite her insisting that he sell it and use the money to get on in life. He was having none of it! Good old biff.