(Promoted from a comment. This answer assumes that the application for the immigrant visa is still pending, though it may also be applicable if the immigrant application was rejected. I welcome further comments to help me improve this answer.)
If the application for an immigrant visa is still pending, then it's unlikely that a visitor visa would be granted. Applicants for visitor visas to the US have to overcome a "presumption of immigrant intent." This means that when your sister applied for a visitor visa, the immigration officers who evaluate the application will start with the presumption that she is really trying to come to live in the US permanently, and it's your sister's burden to prove that this is not the case. Your sister obviously does have immigrant intent, since she has a pending application for an immigrant visa; and you should assume that the immigration officers will be aware of this other application.
If the the terms of your residency in the US allow you to enter and leave the country freely, you're probably better off visiting your sisters in Canada during your older sister's visit.