I don't think Claire sells the house only for financial reasons. Claire and her boyfriend both have good jobs, so it isn't that she needs the money. Now that Robert has died, Claire feels responsible for Catherine, wanting her to move to New York with her and her boyfriend. Claire has worked extra hard to pay off the mortgage to the house because she knew her mentally-ill father wouldn't be able to keep up the payments. Claire allowed her father to stay in the family home by making the payments, and now that he's dead, she is afraid that Catherine may have inherited the mental illness of their father. She feels she needs to take care of Catherine, just as Catherine took care of Robert. By selling the house, Catherine thinks Claire will have no choice but to move with her. If she doesn't sell the house, Catherine will stay there, and Claire will not be able to look after her. Claire is afraid to leave Catherine alone.