Depends on your status. Are you Canadian citizen or PR? If you are either of them then they'll think because your sister has pretty strong ties in Canada, she might not be a bona fide student. Your sister can write a strong SOP describing: her reason why she's going to study in Canada; her strong ties to her home country; her future plan etc.
I'm not sure but I think she can state on an affidavit that she will return back home once her studies are over. Getting something notarized will provide a strong proof to CIC and they can send your sister back if she goes against it and will treat this as misinterpretation.