It's sad to admit but I'd do the same if I was her. She's being sent off to what amounts to solitary confinement and if she's expected to bare the children of a man she doesn't know, can't see and can't give consent to, she's justified in taking extreme measures.
Her father should be ashamed of himself. Sending his daughter off to, presumably, be used as a baby factory by someone she doesn't love. At best, she'd be a kept doll that goes untouched by the guy. That's not a life.
She has nekomori to help her. Even if they struggled with money after the dad is dead, it would still be better than this.
MC and nekomori should just take the kid and run.