Is her shadow slipping off from her or is that her sister's shadow?

It's the bracelet

It's none of those things. The shadows are the left over powers of all the gray/silver haired kids that actually did get sacrificed that manifested in Leslie.

Nah I think you misread the question. We all know where her powers came from. The question is why was it struggling so much to work, check the last page.

At least that's what I got, cause everyone should know where her powers came from by now.
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Ah so it was her power, but it was struggling to come out. I thought it was weird how she almost tripped herself because of it.

I'm glad that wasn't her sister's

The bracelet on her wrist was preventing her powers from working, I think chains that her father had are somehow in the bracelet. It hasn't been clarified by the story as of yet so this is just a theory.

That's exactly what I believe. He did make mention of it before that it was still in his possession