the cracked cylinder heads went into p-40 planes , we know larry never flew that kind of plane,, what does kate mean when she says"your brother's alive,darling,because if he is ead ,your father killed him ?

The last part of the quote above reads "Do you understand me now?" In order to really understand Kate's meaning you have to look at her statement in context. Kate seems to be very aware of Keller's business dealings. It's quite possible she was privy to the shipment of defective cylinders and looked other way. To condemn her husband, all while admitting that Larry was dead might have put some of the guilt on her as well.