I think it was @softlyraining who mentioned it in the thread for Week 4 (could be misremembering), but I was thinking the same. First off, I partially think it because of how prominent role she is playing in the story: narrative-wise, I think it makes sense that the person who dies would be someone who the reader gets to know: to me that leaves the narrator and Misaki as obvious choices just from a narrative perspective. It certainly could be the narrator, but that feels like the more obvious choice to me. There would a sort of macabre tragedy in seeing one of the most vocal supporters of the system being the one who dies, which is why I think it’s definitely possible.
But also, the story talks about how the sister meet Misaki while coming at home after several miscarriages, and how much Misaki took to her. With the sister, it’s established that she doesn’t necessarily want to kill because of clear need for revenge: rather that it’s a dark urge that she feels like she can’t escape, hence the notebook. To me, killing someone who is innocent seems like a better fit for satisfying that darkness, even if the case might be the feeling that is somewhat neglected by her mother.
Obviously Misaki wouldn’t have been the original intended victim as she wouldn’t have been born when she first became an 産み人 but perhaps when she came home she was on the fence: but then meeting Misaki she found the perfect victim, the one thing that might finally put an end to the dark urges she felt once and for all.
But this is also just wild speculation! So I could be completely off.