It ain't Nene's fault in the sense that she's never been malicious toward Nana, but...yeah. It's particularly knife-twisting how earnestly enthusiastic she's being in her "consideration" toward her sister, since you can't even really blame her for any of it.
But, Mirei obviously needn't suffer Nanas misplaced wrath, though, so hopefully Emi being there is either a deterrent or a means of stopping imminent violence, and hopefully soon Nana will get the chance to finally speak her mind to her sister about the lifelong pain she's felt.