Well, then it will be up to you to maintain your sense of self and not slink away when she's there. You will have to step up your efforts to have a polite relationship with her, and if she is dismissive of you do not let it go by. You will have to stand your ground as someone who has a place in his life. If there are things you won't stand for, don't let them get away with shit.
Wait - who will sleep on the couch when you are there - you or her? I thought she was moving in, but into a spare room. She doesn't have to move in and into his bedroom, does she? Isn't there a space in the house she can designate as her room, separate from his?