Hi,
I have been in a very similar situation.
As I understand, your sister applied for a student visa? For what kind of study? Are we taking about a student visa to take just a simple course (english, IT,...) for just some months?
Some possible options:
-If she applies for a student visa for undergraduate or graduate study in a Canadian university, then she will have better options to get a visa (I believe so). Did you explore that already?
-Another alternative will be to withdraw her from your mom's application as dependent and try again for a student visa. Did you try that?
I don't advise her to apply for a tourist visa now. I believe it will be probably denied. Sorry
I went through almost same situation. If you want we could communicate in a direct way (I am not an attorney or anything like that). Let me know if you are interested to send you my email address.
Good luck...