So sorry about your brother. I hope that the cancer can be contained and removed. I lived through my father's death from cancer many years ago. I still carry him with me everywhere I go and talk to him when times get tough.
I know you don't want to hear this now, but you have not been replaced. You cannot be replaced. Living with this woman will simply cause the glasses to come off sooner and for him to really see her. I don't think it will matter for you as a couple, because I don't think that is what you want, but it may bring you some peace about the reality of the last three decades.
