I think it's been 46 chapters since we seen this fine lady. Is she a random NPC or a potential companion for the future? Doesn't look like the sister to me.
She does fit Sana's characteristics considering we've never seen Sana with untied hair before.
We're unable to pinpoint exactly when this scene takes place on the timeline outside of a vague "close to the ritual" but we can assume it takes place on the day or the day prior as it wouldn't make sense to have the sacrifice already tied up earlier than that.
We also know Sana was at the Capital right after Yukito's usurpation in Ch.2, Page 7.
Yukito has been in Everworld for only a week as of Ch.43, Page 3 which is where we were informed that he only has 6 days left before the ritual and that it would take one day to reach Winterhaven (This is why the time it took to break the door was changed from 12 to 8 hour in the physical release) and only one day has passed since they arrived in Winterhaven, which means there should be 4 days remaining before the ritual.
So, this scene should be taking place 12~13 days after Sana's last known location which means there was indeed enough time for her to be transported there if she was indeed kidnapped by the Temple of Evil.
Considering the "Great Invasion" was described to us as having plunged the Royal Capital into "Chaos" recently, it could very well end up revealed to be an attack led by the Temple of Evil's minions to capture Sana and use her as a vessel for Nephilia.
In conclusion, everything points to this woman being Sana but it could also be some random woman if the author decides to prolong the series.
Fun fact because I took way too long skimming through every chapter to get a full timeline,
Riverna has only known Yukito for two whole days, Lumilia for 4/5 days and Charlotte at the top with 9 days!