I don't particularly trust her.
Her introduction made her seem very well aware of very general social manipulation, which may not have been malicious, but it is still one of those elements of a person being aware of how to maximize the benefits of a social interaction. Pair that up with her cowardice, which is understandable, but still shows that she is far more likely to be a flight than fight.
She seems set up for either redemption or for creating a worsening circumstance in the near future. Hell, for all we know she'll be the cause for their death and we'll loop again which will create more tense relations between the MC and this girl on subsequent loops. To which, of course, his sister will try to make sure to maintain his humanity and general attachment to other humans outside of just herself. But that's of course if she wrecks things.
After all, the threat in this story isn't the MC dying. We know what happens when he dies. The threat is if this loop succeeds or fails. And the further he keeps on chaining success, the worse it could be when failure comes roaring in.