It's either incest or it's not.
They're either biologically related and it's incest or they are not and it's fine. There's no "pseudo incest".
I mean it would be a bit weirder if they were together since they were born as siblings in the same family, then they would most likely have a lot of...
Why the "bad" effect of the hourglass is mathematically negligible:
[spoiler]
She can use the hourglass once per day to turn back 5 minutes.
It seems that she ages during these 5 minutes extra time.
She only loses:
5 minutes * 365 = 1.267 days
So she loses only 1.267 days per year if she uses...