You need to somehow check where you buried her without anyone noticing. Especially your “sister who returned”. If the body is still there, then it’s probably a changeling or a skinwalker, or someone just masquerading as her. If not, then someone or something has resurrected her. I’d seek help with either of these scenarios, but make sure that you are never alone with your sister. We have no idea how she’ll react, so it’s best to always be with someone else. Also make sure you carry a pocketknife or some kind of weapon in case she tries to attack you.

I that it’s just someone masquerading, but I suspect not. Hope this helps you out OP.