Interim work authorization is a permit for 180 days or until they make a decision on your file in my case I do believe just because they see humanitarian reasons or my H&C is strong and it's about to get approved 1 stage they can issue that work authorization or because the application is very delayed I do believe they can send this work authorization.
Now if your sister it's not in Canada for more than 3 years the file I'll be refuse if she doesn't work and you helper financial they're file I'll be refuse because immigration at some point I'll ask for bank statements and if she doesn't have money or income they refuse because of financial reasons immigration don't want your sister to get government assistance if she doesn't have kids with her under 16 file I'll get refuse ,if she doesn't have Canadian or permanent resident family here file I'll get refuse, if she as more family back home then in Canada file I'll be refuse because she as more support back home,if she doesn't have a humanitarian reason to stay in Canada they I'll send you back home and from there you can applied for other immigration programs, now if shes from Gaza or Ukraine or a very very very poor country or something like that probably she as some chances to get approved.
I applied same time as your sister my file as around 1700 pages it's been 21 month 10 months was just the file sitting there waiting for someone to work on the file and after 21 month I got the interim work authorization at this moment I don't know nothing else. I'm so sorry if I was to rude and to honest the best to your family I hope they can get what they looking for.
