Best is to check the government website on covid travel restrictions. It seems to me pretty clear that immediate family members can enter with COPR/visa. As I understand your case, she is travelling as dependent/immediate family of the parents, not as sibling. So I don't see why consultants are confused.
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The confusion is because the parents haven’t received PR yet so the sibling would not be traveling as the child of a PR. There was another case (
@ZOYAMAN13 ) whose brother was able to travel with his parents as apart of PGP with the exact same situation. I believe that there is more flexibility because now extended families can travel before the sibling, as a part of a PGP Application, could technically only come to Canada if they received COPR before a March 18th or when the parents received PR.